Bug 1178718. Part 2 - No need to acquire the monitor in BreakCycles(). r=roc.

dom/media/MediaDecoderStateMachine.h

@@ -320,10 +320,7 @@ public:
     if (mReader) {
       mReader->BreakCycles();
     }
-    {
-      ReentrantMonitorAutoEnter mon(mDecoder->GetReentrantMonitor());
     mDecodedStream.DestroyData();
-    }
     mDecoder = nullptr;
   }