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

chromecast/media/audio/cast_audio_manager.h

Index: chromecast/media/audio/cast_audio_manager.h
diff --git a/chromecast/media/audio/cast_audio_manager.h b/chromecast/media/audio/cast_audio_manager.h
index dc8c7d5c262a9df1183e85b432d3819f7409a507..f2cec7e1d556e0ef6f00e8a4889812546af44d99 100644
--- a/chromecast/media/audio/cast_audio_manager.h
+++ b/chromecast/media/audio/cast_audio_manager.h
@@ -15,14 +15,11 @@
 namespace media {
 
 class MediaPipelineBackend;
-class MediaPipelineBackendManager;
 struct MediaPipelineDeviceParams;
 
 class CastAudioManager : public ::media::AudioManagerBase {
  public:
-  // This class does not take ownership of |backend_manager|
-  CastAudioManager(::media::AudioLogFactory* audio_log_factory,
-                   MediaPipelineBackendManager* backend_manager);
+  explicit CastAudioManager(::media::AudioLogFactory* audio_log_factory);
   ~CastAudioManager() override;
 
   // AudioManager implementation.
@@ -56,8 +53,6 @@
       const std::string& output_device_id,
       const ::media::AudioParameters& input_params) override;
 
-  MediaPipelineBackendManager* const backend_manager_;
-
   DISALLOW_COPY_AND_ASSIGN(CastAudioManager);
 };