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.

---
<!-- 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/messageevent.rs

@@ -16,7 +16,6 @@ use dom::globalscope::GlobalScope;
 use js::jsapi::{HandleValue, Heap, JSContext};
 use js::jsval::JSVal;
 use servo_atoms::Atom;
-use std::default::Default;
 
 #[dom_struct]
 pub struct MessageEvent {
@@ -38,13 +37,12 @@ impl MessageEvent {
                            data: HandleValue,
                            origin: DOMString,
                            lastEventId: DOMString) -> Root<MessageEvent> {
-        let mut ev = box MessageEvent {
+        let ev = box MessageEvent {
             event: Event::new_inherited(),
-            data: Heap::default(),
+            data: Heap::new(data.get()),
             origin: origin,
             lastEventId: lastEventId,
         };
-        ev.data.set(data.get());
         reflect_dom_object(ev, global, MessageEventBinding::Wrap)
     }