Bug 1282992 - Improve sync event loop shutdown in workers, r=khuey

dom/workers/WorkerPrivate.cpp

@@ -5515,8 +5515,8 @@ WorkerPrivate::DestroySyncLoop(uint32_t aLoopIndex, nsIThreadInternal* aThread)
   MOZ_ALWAYS_SUCCEEDS(aThread->PopEventQueue(nestedEventTarget));
 
   if (mSyncLoopStack.IsEmpty() && mPendingEventQueueClearing) {
-    ClearMainEventQueue(WorkerRan);
     mPendingEventQueueClearing = false;
+    ClearMainEventQueue(WorkerRan);
   }
 
   return result;