Bug 1153370 - Rescope monitor. r=jww

We can do this now that we're not manually nulling out mRunningThread anymore.

dom/media/MediaTaskQueue.cpp

@@ -275,20 +275,14 @@ MediaTaskQueue::Runner::Run()
   // run in a loop here so that we don't hog the thread pool. This means we may
   // run on another thread next time, but we rely on the memory fences from
   // mQueueMonitor for thread safety of non-threadsafe tasks.
-  {
-    MonitorAutoLock mon(mQueue->mQueueMonitor);
-    // Note: Hold the monitor *before* we dispatch, in case we context switch
-    // to another thread pool in the queue immediately and take the lock in the
-    // other thread; mRunningThread could be set to the new thread's value and
-    // then incorrectly anulled below in that case.
   nsresult rv = mQueue->mPool->Dispatch(this, NS_DISPATCH_NORMAL);
   if (NS_FAILED(rv)) {
     // Failed to dispatch, shutdown!
+    MonitorAutoLock mon(mQueue->mQueueMonitor);
     mQueue->mIsRunning = false;
     mQueue->mIsShutdown = true;
     mon.NotifyAll();
   }
-  }
 
   return NS_OK;
 }