Auto merge of #25446 - pshaughn:fixme11868, r=jdm

Make getOwnPropertyDescriptor hold the correct value for indexed/named properties

This is towards #25036 and #25415; it could have far-reaching implications so I want to test it in isolation and see the results on the full test suite.

A few lines of code had a FIXME(#11868) despite that issue being closed, and looking for the pattern that was marked that way, I found one other unmarked instance of it. It doesn't immediately crash, and maybe it will pass some tests or fail some tests in informative ways.

EDIT: After adding an overlooked extended attribute to HTMLFormElement, this works very well indeed and seems to be worth merging!

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components/script/dom/webidls/HTMLFormElement.webidl

@@ -3,7 +3,7 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 // https://html.spec.whatwg.org/multipage/#htmlformelement
-[Exposed=Window]
+[Exposed=Window, LegacyUnenumerableNamedProperties]
 interface HTMLFormElement : HTMLElement {
   [HTMLConstructor] constructor();
 

components/script/dom/webidls/Window.webidl

@@ -3,7 +3,7 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 // https://html.spec.whatwg.org/multipage/#window
-[Global=Window, Exposed=Window]
+[Global=Window, Exposed=Window /*, LegacyUnenumerableNamedProperties */]
 /*sealed*/ interface Window : GlobalScope {
   // the current browsing context
   [Unforgeable] readonly attribute WindowProxy window;