[chromecast] Pass media task runner to MediaPipelineBackendManager.

chromecast/media/audio/cast_audio_manager.cc

Index: chromecast/media/audio/cast_audio_manager.cc
diff --git a/chromecast/media/audio/cast_audio_manager.cc b/chromecast/media/audio/cast_audio_manager.cc
index 54cd79454398beafd1e73e8aa929027aaf3ad4d8..229e2b0cb44ef5dcdbcda8223fe680802e7af1b1 100644
--- a/chromecast/media/audio/cast_audio_manager.cc
+++ b/chromecast/media/audio/cast_audio_manager.cc
@@ -28,8 +28,10 @@ static const int kDefaultOutputBufferSize = 2048;
 namespace chromecast {
 namespace media {
 
-CastAudioManager::CastAudioManager(::media::AudioLogFactory* audio_log_factory)
-    : AudioManagerBase(audio_log_factory) {}
+CastAudioManager::CastAudioManager(::media::AudioLogFactory* audio_log_factory,
+                                   MediaPipelineBackendManager* backend_manager)
+    : AudioManagerBase(audio_log_factory), backend_manager_(backend_manager) {
+}
 
 CastAudioManager::~CastAudioManager() {
   Shutdown();
@@ -64,10 +66,7 @@ void CastAudioManager::GetAudioInputDeviceNames(
 
 scoped_ptr<MediaPipelineBackend> CastAudioManager::CreateMediaPipelineBackend(
     const MediaPipelineDeviceParams& params) {
-  DCHECK(media::MediaMessageLoop::GetTaskRunner()->BelongsToCurrentThread());
-
-  return scoped_ptr<MediaPipelineBackend>(
-      MediaPipelineBackendManager::CreateMediaPipelineBackend(params));
+  return backend_manager_->CreateMediaPipelineBackend(params);
 }
 
 ::media::AudioOutputStream* CastAudioManager::MakeLinearOutputStream(