Bug 1360123 P3 - while shutting down a MediaDecoder, ask its owner to remove all media tracks; r=jwwang

MozReview-Commit-ID: 6s84iyXJQ9k

--HG--
extra : rebase_source : 443d6fafd8c17d02dba6ba0054abbb473079545a

dom/media/MediaDecoder.cpp

@@ -503,6 +503,9 @@ MediaDecoder::Shutdown()
     mResource->Close();
   }
 
+  // Ask the owner to remove its audio/video tracks.
+  GetOwner()->RemoveMediaTracks();
+
   ChangeState(PLAY_STATE_SHUTDOWN);
   mOwner = nullptr;
 }