Auto merge of #15120 - asajeffrey:script-windows-share-browsing-contexts, r=jdm

Allow windows to share browsing contexts.

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This PR allows different `Window` objects in the same browsing context to share a `BrowsingContext` object.

SpiderMonkey requires a `WindowProxy` object to be in the same compartment as its `Window`, so when a `WindowProxy` changes `Window`, we have to brain-transplant it. In turn this requires the reflector of a `BrowsingContext` to be mutable.

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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 #13608 and #14843
- [X] These changes do not require tests because an existing test catches this (`/html/browsers/the-window-object/Window-document.html` is now `PASS`)

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tests/wpt/metadata/html/browsers/the-window-object/Window-document.html.ini

@@ -1,5 +0,0 @@
-[Window-document.html]
-  type: testharness
-  [Document in a browsing context]
-    expected: FAIL
-