Bug 1809753 - Part 4: Remove unnecessary GetMainThreadEventTarget, r=mccr8

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

dom/websocket/WebSocket.cpp

@@ -2755,7 +2755,7 @@ WebSocketImpl::Dispatch(already_AddRefed<nsIRunnable> aEvent, uint32_t aFlags) {
   if (mIsMainThread) {
     return mMainThreadEventTarget
                ? mMainThreadEventTarget->Dispatch(event_ref.forget())
-               : GetMainThreadEventTarget()->Dispatch(event_ref.forget());
+               : GetMainThreadSerialEventTarget()->Dispatch(event_ref.forget());
   }
 
   MutexAutoLock lock(mMutex);