Only install @@toStringTag on interface prototype objects

This fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=28244,
aligning Web IDL objects with the built-in ECMAScript objects (see
#226), and updates the spec to match 1/3 engines (Chromium) instead of
0/3 as it currently does. The other two engines are willing to align,
however!

Tests: web-platform-tests/wpt#23140

Author: domenic

index.bs

@@ -7175,7 +7175,7 @@ Unless otherwise specified, the \[[Prototype]] internal slot
 of objects defined in this section is {{%ObjectPrototype%}}.
 
 Some objects described in this section are defined to have a <dfn id="dfn-class-string" export>class string</dfn>,
-which is the string to include in the string returned from Object.prototype.toString.
+which is the string to include in the string returned from <code>Object.prototype.toString</code>.
 
 If an object has a [=class string=] |classString|, then the object must,
 at the time it is created, have a property whose name is the {{@@toStringTag}} symbol
@@ -11351,8 +11351,7 @@ Issue: Define those properties imperatively instead.
 
 </div>
 
-The [=class string=] of an [=interface prototype object=] is the concatenation of
-the [=interface=]’s [=qualified name=] and the string "<code>Prototype</code>".
+The [=class string=] of an [=interface prototype object=] is the [=interface=]’s [=qualified name=].
 
 
 <h4 id="named-properties-object">Named properties object</h4>
@@ -12241,7 +12240,7 @@ restricted to iterating over an object’s
 [=support indexed properties|supported indexed properties=],
 use standard ECMAScript Array iterator objects.
 
-[=Default iterator objects=] do not have [=class strings=]; when <code
+Note: [=Default iterator objects=] do not have [=class strings=]; when <code
 class="idl">Object.prototype.toString()</code> is called on a [=default
 iterator object=] of a given [=interface=], the [=class string=] of the
 [=iterator prototype object=] of that [=interface=] is used.
@@ -13170,12 +13169,6 @@ updated to be the [=interface prototype object=]
 of the [=primary interface=]
 from the [=platform object=]’s newly associated [=Realm=].
 
-The [=class string=] of
-a platform object that implements one or more interfaces
-must be the [=qualified name=] of
-the [=primary interface=]
-of the platform object.
-
 Additionally, [=platform objects=] which implement an [=interface=]
 which has a [{{Global}}] [=extended attribute=]
 get properties declaratively from: