WindowProxy property access can reach COOP access reporting after iframe
removal. That leaves the accessed active document not fully active.
The reporting algorithm only applies to fully active documents. Return
early for inactive documents instead of asserting. Add a crash test for
accessing a window proxy after iframe removal.
In WindowProxy.[[Get]] it's not guaranteed that the current principal
global object has an associated document at the moment. This may happen
if a script is continuing to execute while a navigation has been
initiated.
Because of that, we can't blindly dereference the active document
pointer, so this patch adds a null check.
