Bug 868406. Shut down MediaStreamGraph's thread. r=jesup

--HG--
extra : rebase_source : 0d5801b271d1658dba172cb5b156186052651990

content/media/MediaStreamGraph.cpp

@@ -1139,6 +1139,8 @@ public:
     NS_ASSERTION(mGraph->mDetectedNotRunning,
                  "We should know the graph thread control loop isn't running!");
 
+    mGraph->ShutdownThreads();
+
     // mGraph's thread is not running so it's OK to do whatever here
     if (mGraph->IsEmpty()) {
       // mGraph is no longer needed, so delete it. If the graph is not empty