emulation.setLocaleOverride (#924)

Addressing #774. 

Emulate locales in [`Intl`](https://tc39.es/ecma402/#intl-object). Other i18n emulation is out of scope of this PR. 

Emulate locale for browsing contexts or for user contexts.

The ecmascript specification of [Intl](https://tc39.es/ecma402/#intl-object) is tight to [DefaultLocale()](https://tc39.es/ecma402/#sec-defaultlocale), which is an [implementation-defined](https://tc39.es/ecma262/#implementation-defined) abstract operation. This PR defines additional steps in case of the locale override is set.

Author: sadym-chromium

index.bs

@@ -127,6 +127,10 @@ spec: CONSOLE; urlPrefix: https://console.spec.whatwg.org
     text: formatter; url: formatter
     text: formatting specifier; url: formatting-specifiers
     text: printer; url: printer
+spec: ECMASCRIPT-I18N; urlPrefix: https://tc39.es/ecma402/
+  type: dfn
+    text: DefaultLocale; url: #sec-defaultlocale
+    text: IsStructurallyValidLanguageTag; url: #sec-isstructurallyvalidlanguagetag
 spec: ECMASCRIPT; urlPrefix: https://tc39.es/ecma262/
   type: dfn
     text: Array; url: sec-array-objects
@@ -3116,6 +3120,9 @@ or null and an [=struct/item=] named <dfn attribute for="viewport-configuration"
 
 A [=remote end=] has a <dfn>viewport overrides map</dfn> which is a weak map between [=user contexts=] and [=viewport configuration=].
 
+A [=remote end=] has a <dfn>locale overrides map</dfn> which is a weak map between
+[=navigables=] or [=user contexts=] and string or null.
+
 ### Types ### {#module-browsingcontext-types}
 
 #### The browsingContext.BrowsingContext Type #### {#type-browsingContext-Browsingcontext}
@@ -5768,6 +5775,7 @@ relating to emulation of browser APIs.
 <pre class="cddl" data-cddl-module="remote-cddl">
 EmulationCommand = (
   emulation.SetGeolocationOverride //
+  emulation.SetLocaleOverride //
   emulation.SetScreenOrientationOverride
 )
 </pre>
@@ -5898,6 +5906,125 @@ The [=remote end steps=] with |command parameters| are:
 
 </div>
 
+#### The emulation.setLocaleOverride Command ####  {#command-emulation-setLocaleOverride}
+
+The <dfn export for=commands>emulation.setLocaleOverride</dfn> command modifies
+locale on the given top-level traversables or user contexts.
+
+<dl>
+   <dt>Command Type</dt>
+   <dd>
+    <pre class="cddl" data-cddl-module="remote-cddl">
+      emulation.SetLocaleOverride = (
+        method: "emulation.setLocaleOverride",
+        params: emulation.SetLocaleOverrideParameters
+      )
+
+      emulation.SetLocaleOverrideParameters = {
+        locale: text / null,
+        ? contexts: [+browsingContext.BrowsingContext],
+        ? userContexts: [+browser.UserContext],
+      }
+    </pre>
+   </dd>
+   <dt>Result Type</dt>
+   <dd>
+    <code>
+     EmptyResult
+    </code>
+   </dd>
+</dl>
+
+<div algorithm="updated DefaultLocale steps">
+The [=DefaultLocale=] algorithm is implementation defined. A WebDriver-BiDi
+[=remote end=] must have an implementation that runs the following steps:
+
+1. Let |emulated locale| be null.
+
+1. Let |realm| be [=current Realm Record=].
+
+1. Let |global object| be |realm|'s [=realm/global object=].
+
+1. If |global object| implements {{WindowProxy}}:
+
+    1. Let |window| be the value of |global object|'s \[[WindowProxy]]
+       [=internal slot=].
+
+    1. Let |navigable| be |window|'s [=window/navigable=].
+
+    1. Let |top-level traversable| be |navigable|’s
+       [=navigable/top-level traversable=].
+
+    1. Let |user context| be |top-level traversable|'s [=associated user context=].
+
+    1. If [=locale overrides map=] [=map/contains=] |top-level traversable|, set
+       |emulated locale| to [=locale overrides map=][|top-level traversable|].
+
+    1. Otherwise, if [=locale overrides map=] [=map/contains=]
+       |user context|, set |emulated locale| to
+       [=locale overrides map=][|user context|].
+
+1. If |emulated locale| is not null, return |emulated locale|.
+
+1. Return the result of implementation-defined steps in accordance with the
+   requirements of the [=DefaultLocale=] specification.
+
+</div>
+
+<div algorithm="remote end steps for emulation.setLocaleOverride">
+
+The [=remote end steps=] with |command parameters| are:
+
+1. If |command parameters| [=map/contains=] "<code>userContexts</code>"
+   and |command parameters| [=map/contains=] "<code>context</code>",
+   return [=error=] with [=error code=] [=invalid argument=].
+
+1. If |command parameters| doesn't [=map/contain=] "<code>userContexts</code>"
+   and |command parameters| doesn't [=map/contain=] "<code>context</code>",
+   return [=error=] with [=error code=] [=invalid argument=].
+
+1. Let |emulated locale| be null.
+
+1. If |command parameters| [=map/contains=] "<code>locale</code>":
+
+   1. Set |emulated locale| to |command parameters|["<code>locale</code>"].
+
+   1. If [=IsStructurallyValidLanguageTag=](|emulated locale|) returns false,
+      return [=error=] with [=error code=] [=invalid argument=].
+
+1. Let |navigables| be a [=/set=].
+
+1. If the <code>contexts</code> field of |command parameters| is present:
+
+   1. Let |navigables| be the result of [=trying=] to
+      [=get valid top-level traversables by ids=] with
+      |command parameters|["<code>contexts</code>"].
+
+1. Otherwise:
+
+   1. Assert the <code>userContexts</code> field of |command parameters| is present.
+
+   1. Let |user contexts| be the result of [=trying=] to [=get valid user contexts=]
+      with |command parameters|["<code>userContexts</code>"].
+
+   1. For each |user context| of |user contexts|:
+
+      1. [=map/Set=] [=locale overrides map=][|user context|] to |emulated locale|.
+
+      1. [=list/For each=] |top-level traversable| of the list of all
+         [=/top-level traversables=] whose [=associated user context=] is
+         |user context|:
+
+         1. [=list/Append=] |top-level traversable| to |navigables|.
+
+1. For each |navigable| of |navigables|:
+
+   1. [=map/Set=] [=locale overrides map=][|navigable|] to |emulated locale|.
+
+1. Return [=success=] with data null.
+
+</div>
+
 #### The emulation.setScreenOrientationOverride Command ####  {#command-emulation-setScreenOrientationOverride}
 
 The <dfn export for=commands>emulation.setScreenOrientationOverride</dfn> command