[Chromecast] Add/Remove streams on Media Thread

Fix threading issue when adding and removing streams.

Change-Id: Ib2ff6ed87bc7d96a2d0956c604f741be38992c7c
Reviewed-on: https://chromium-review.googlesource.com/1220555Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Jimmy Tobin <jtobin@google.com>
Cr-Commit-Position: refs/heads/master@{#590533}

chromecast/media/cma/backend/media_pipeline_backend_manager.cc

@@ -185,12 +185,14 @@ void MediaPipelineBackendManager::RemoveAllowVolumeFeedbackObserver(
 void MediaPipelineBackendManager::AddExtraPlayingStream(
     bool sfx,
     const AudioContentType type) {
+  MAKE_SURE_MEDIA_THREAD(AddExtraPlayingStream, sfx, type);
   UpdatePlayingAudioCount(sfx, type, 1);
 }
 
 void MediaPipelineBackendManager::RemoveExtraPlayingStream(
     bool sfx,
     const AudioContentType type) {
+  MAKE_SURE_MEDIA_THREAD(RemoveExtraPlayingStream, sfx, type);
   UpdatePlayingAudioCount(sfx, type, -1);
 }