We want to be able to enter the Realm we were in when the callback was created
before calling it, but if the callback stores a cross-compartment wrapper we
don't really have a good way to find that Realm. So we store it explicitly by
storing a global when the callback is created.
The changes to the constructor signatures to use JSObject* instead of
JS::Handle<JSObject*> are so we can avoid having to root the global for these
calls. These changes make two of the constructors ambiguous when nullptr is
being passed for the first arg; this patch adds casts to disambiguate.
Boris Zbarsky
Jan Henning