Bug 1109437 - Schedule the state machine thread in MediaSourceDecoder::Load. r=cpearce

This is necessary to get the state machine moving after it's initialized.
MediaDecoder::Load does this, but it looks like we missed this in the override.
So the current code relies on the ScheduleStateMachine call at the end of
TrackBuffer::AppendData to get things rolling. We're going to be removing that
call, so we need to fix this.

dom/media/mediasource/MediaSourceDecoder.cpp

@@ -66,8 +66,7 @@ MediaSourceDecoder::Load(nsIStreamListener**, MediaDecoder*)
   NS_ENSURE_SUCCESS(rv, rv);
 
   SetStateMachineParameters();
-
-  return NS_OK;
+  return ScheduleStateMachineThread();
 }
 
 nsresult