Bug 1135785 - Stop invoking StopPlayback in SetDormant. r=cpearce

This already gets incoded in the DECODER_STATE_DORMANT case of RunStateMachine,
which will run momentarily on the state machine thread. Doing this allows us to
avoid calling StopPlayback on the main thread.

dom/media/MediaDecoderStateMachine.cpp

@@ -1511,7 +1511,6 @@ void MediaDecoderStateMachine::SetDormant(bool aDormant)
     mCurrentSeekTarget.Reset();
     ScheduleStateMachine();
     SetState(DECODER_STATE_DORMANT);
-    StopPlayback();
     mDecoder->GetReentrantMonitor().NotifyAll();
   } else if ((aDormant != true) && (mState == DECODER_STATE_DORMANT)) {
     mDecodingFrozenAtStateDecoding = true;