Bug 1541557: Part 1 - Use correct globals for JSWindowActors not in the shared JSM global. r=nika The current JSWindowActor code assumes that all actors will be created in the shared JSM global. This has a few problems: 1) For actors in other scopes, it enters the wrong compartment before decoding messages, which leads to a compartment checker error when trying to call message handlers. That could be fixed by just wrapping the result for the caller, but that would lead to other problems. Aside from the efficiency concerns of having to deal with cross-compartment wrappers, SpecialPowers in particular would not be able to pass the resulting objects to unprivileged scopes, since only SpecialPowers compartments have permissive CCWs enabled. 2) It also leads to the prototype objects for the actor instances being created in the shared JSM scope, even when the actors themselves are defined in other compartments. Again, aside from CCW efficiency issues, this prevents the SpecialPowers instances from being accessed by the unprivileged scopes that they're exposed to, since the prototype objects live in privileged scopes which don't have permissive CCWs enabled. This patch changes child actors to always create their objects in the global of their constructors. The parent objects are still created in the shared JSM global, but they now wrap values for the appropriate compartment before trying to call message handlers. Differential Revision: https://phabricator.services.mozilla.com/D35051

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