servo: Merge #15120 - Allow windows to share browsing contexts (from asajeffrey:script-windows-share-browsing-contexts); r=jdm

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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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<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [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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Source-Repo: https://github.com/servo/servo
Source-Revision: 67c182638253211553161495cd2e4570002fd5bc

servo/components/script/dom/vreyeparameters.rs

@@ -31,7 +31,7 @@ impl VREyeParameters {
     #[allow(unrooted_must_root)]
     fn new_inherited(parameters: WebVREyeParameters, global: &GlobalScope) -> VREyeParameters {
         let fov = VRFieldOfView::new(&global, parameters.field_of_view.clone());
-        let mut result = VREyeParameters {
+        let result = VREyeParameters {
             reflector_: Reflector::new(),
             parameters: DOMRefCell::new(parameters),
             offset: Heap::default(),