Auto merge of #20329 - gterzian:before_unload, r=cbrewster

Implement beforeunload event and infrastructure

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Implementation of:
1. https://html.spec.whatwg.org/multipage/#prompt-to-unload-a-document, and
2. https://html.spec.whatwg.org/multipage/#unload-a-document
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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #10787 and fix #20485 and fix #20588 and fix #20496 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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components/script/dom/webidls/Document.webidl

@@ -83,7 +83,7 @@ dictionary ElementCreationOptions {
 // [OverrideBuiltins]
 partial /*sealed*/ interface Document {
   // resource metadata management
-  [/*PutForwards=href, */Unforgeable]
+  [PutForwards=href, Unforgeable]
   readonly attribute Location? location;
   [SetterThrows] attribute DOMString domain;
   readonly attribute DOMString referrer;

components/script/dom/webidls/Window.webidl

@@ -12,7 +12,7 @@
 
   attribute DOMString name;
 
-  [/*PutForwards=href, */Unforgeable] readonly attribute Location location;
+  [PutForwards=href, Unforgeable] readonly attribute Location location;
   readonly attribute History history;
   [Pref="dom.customelements.enabled"]
   readonly attribute CustomElementRegistry customElements;