WebDriver BiDi emulated user-agent

Support for user-agent emulations.

Author: sadym-chromium

fetch.bs

@@ -5889,6 +5889,21 @@ run these steps:
    the <a data-lt="structured field token">token</a> <code>prefetch</code>) in
    <var>httpRequest</var>'s <a for=request>header list</a>.
 
+   <li>
+    <p>If <var>httpRequest</var>'s <a for=request>header list</a> <a for="header list">does not
+    contain</a> `<code>User-Agent</code>`, then:
+
+    <ol>
+     <li><p>Let <var>userAgent</var> be the result of running the [=WebDriver BiDi emulated
+     User-Agent=] with <var>httpRequest</var>'s <a for=request>client</a>.
+
+     <li><p>If <var>userAgent</var> is null, set <var>userAgent</var> to <a>default
+     `<code>User-Agent</code>` value</a>.
+
+     <li><p><a for="header list">Append</a> (`<code>User-Agent</code>`, <var>userAgent</var>) to
+     <var>httpRequest</var>'s <a for=request>header list</a>.
+    </ol>
+
    <li><p>If <var>httpRequest</var>'s <a for=request>header list</a>
    <a for="header list">does not contain</a> `<code>User-Agent</code>`, then user agents should
    <a for="header list">append</a> (`<code>User-Agent</code>`,