This method always returned GetMainThreadSerialEventTarget(). This patch
switches all callers over to use that method instead.
We can't easily switch all calls to be calls to NS_GetMainThread(), as there is
no version of that method returning a bare nsIThread* instance.
I didn't introduce one, as we may want to add a lock around mMainThread in the
future, which would require removing nsThreadManager::GetMainThreadWeak. As
this method only returns nsISerialEventTarget, it method could remain
implemented, however, by returning a statically allocated fake event target
which forwards dispatches (and QIs to nsIThread) to the real main thread.
Differential Revision: https://phabricator.services.mozilla.com/D166608
