Auto merge of #14559 - asajeffrey:script-track-document-and-bc-discarding, r=cbrewster

Implement browsing context discarding

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Implement browsing context discarding (https://html.spec.whatwg.org/multipage/browsers.html#discard-a-document).

* When a pipeline is closed, inform the script thread whether the browsing context is to be discarded.
* In script threads, synchronously discard any similar-origin documents and browsing contexts.
* When a browsing context is discarded, it loses the reference to the active document, but the window keeps it, so we need to move the `Document` pointer from `BrowsingContext` to `Window`.
* Fix the webIDL for Window to make parent and top optional (the spec says they can return null when the browsing context is discarded).

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #14411
- [X] There are tests for these changes

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components/script/dom/webidls/Window.webidl

@@ -31,9 +31,13 @@
   // other browsing contexts
   [Replaceable] readonly attribute WindowProxy frames;
   //[Replaceable] readonly attribute unsigned long length;
-  [Unforgeable] readonly attribute WindowProxy top;
+  // Note that this can return null in the case that the browsing context has been discarded.
+  // https://github.com/whatwg/html/issues/2115
+  [Unforgeable] readonly attribute WindowProxy? top;
   //         attribute any opener;
-  readonly attribute WindowProxy parent;
+  // Note that this can return null in the case that the browsing context has been discarded.
+  // https://github.com/whatwg/html/issues/2115
+  readonly attribute WindowProxy? parent;
   readonly attribute Element? frameElement;
   //WindowProxy open(optional DOMString url = "about:blank", optional DOMString target = "_blank",
   //                 optional DOMString features = "", optional boolean replace = false);