Revert of [chromecast] Pass media task runner to MediaPipelineBackendManager.

chromecast/media/audio/cast_audio_manager_factory.h

Index: chromecast/media/audio/cast_audio_manager_factory.h
diff --git a/chromecast/media/audio/cast_audio_manager_factory.h b/chromecast/media/audio/cast_audio_manager_factory.h
index 627a68dd991e197e8c48d50cfbb68e96d8287a40..85e816001b63545e6357c1d7c5eb1dfb70aa2489 100644
--- a/chromecast/media/audio/cast_audio_manager_factory.h
+++ b/chromecast/media/audio/cast_audio_manager_factory.h
@@ -11,12 +11,9 @@
 namespace chromecast {
 namespace media {
 
-class MediaPipelineBackendManager;
-
 class CastAudioManagerFactory : public ::media::AudioManagerFactory {
  public:
-  explicit CastAudioManagerFactory(
-      MediaPipelineBackendManager* backend_manager);
+  CastAudioManagerFactory();
   ~CastAudioManagerFactory() override;
 
   // ::media::AudioManagerFactory overrides.
@@ -24,8 +21,6 @@
       ::media::AudioLogFactory* audio_log_factory) override;
 
  private:
-  MediaPipelineBackendManager* const backend_manager_;
-
   DISALLOW_COPY_AND_ASSIGN(CastAudioManagerFactory);
 };