Enumerate fewer cross-origin properties

In 205659f we made all properties on
cross-origin objects enumerable, equivalent to their same-origin object
counterparts.

However, this turned out not be web-compatible. This makes them
non-enumerable again with the exception of array index property names,
which need to be enumerable.

Tests: web-platform-tests/wpt#8045

Fixes #3183.

source

@@ -77841,7 +77841,7 @@ console.assert(iframeWindow.frameElement === null);
 
          <li><p>Set <var>crossOriginDesc</var> to <span>PropertyDescriptor</span>{
          [[Value]]: <var>value</var>,
-         [[Enumerable]]: true,
+         [[Enumerable]]: false,
          [[Writable]]: false,
          [[Configurable]]: true }.</p></li>
         </ol>
@@ -77868,7 +77868,7 @@ console.assert(iframeWindow.frameElement === null);
          <li><p>Set <var>crossOriginDesc</var> to <span>PropertyDescriptor</span>{
          [[Get]]: <var>crossOriginGet</var>,
          [[Set]]: <var>crossOriginSet</var>,
-         [[Enumerable]]: true,
+         [[Enumerable]]: false,
          [[Configurable]]: true }.</p></li>
         </ol>
        </li>
@@ -77894,6 +77894,10 @@ console.assert(iframeWindow.frameElement === null);
   elsewhere in this specification for cases where we are not able to preserve these invariants, for
   compatibility with existing Web content.) <ref spec=JAVASCRIPT></p>
 
+  <p class="note">The reason the property descriptors are non-enumerable, despite this mismatching
+  the same-origin behavior, is for compatibility with existing Web content. See <a
+  href="https://github.com/whatwg/html/issues/3183">issue #3183</a> for details.</p>
+
   <h5><dfn>CrossOriginGet</dfn> ( <var>O</var>, <var>P</var>, <var>Receiver</var> )</h5>
 
   <ol>
@@ -79076,11 +79080,17 @@ interface <dfn>BarProp</dfn> {
      <li><p>Let <var>value</var> be the <code>WindowProxy</code> object of the
      <span data-x="dom-window-nameditem-filter">named object</span> with the name <var>P</var>.
 
-     <li><p>Return <span>PropertyDescriptor</span>{
-     [[Value]]: <var>value</var>,
-     [[Enumerable]]: true,
-     [[Writable]]: false,
-     [[Configurable]]: true }.</p></li>
+     <li>
+      <p>Return <span>PropertyDescriptor</span>{
+      [[Value]]: <var>value</var>,
+      [[Enumerable]]: false,
+      [[Writable]]: false,
+      [[Configurable]]: true }.</p>
+
+      <p class="note">The reason the property descriptors are non-enumerable, despite this
+      mismatching the same-origin behavior, is for compatibility with existing Web content. See <a
+      href="https://github.com/whatwg/html/issues/3183">issue #3183</a> for details.</p>
+     </li>
     </ol>
 
    <li><p>Throw a <span>"<code>SecurityError</code>"</span> <code>DOMException</code>.</p></li>