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

chromecast/media/audio/cast_audio_manager_factory.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROMECAST_MEDIA_AUDIO_CAST_AUDIO_MANAGER_FACTORY_H_
 #define CHROMECAST_MEDIA_AUDIO_CAST_AUDIO_MANAGER_FACTORY_H_
 
 #include "base/macros.h"
 #include "media/audio/audio_manager_factory.h"
 
 namespace chromecast {
 namespace media {
 
-class MediaPipelineBackendManager;
-
 class CastAudioManagerFactory : public ::media::AudioManagerFactory {
  public:
-  explicit CastAudioManagerFactory(
-      MediaPipelineBackendManager* backend_manager);
+  CastAudioManagerFactory();
   ~CastAudioManagerFactory() override;
 
   // ::media::AudioManagerFactory overrides.
   ::media::AudioManager* CreateInstance(
       ::media::AudioLogFactory* audio_log_factory) override;
 
  private:
-  MediaPipelineBackendManager* const backend_manager_;
-
   DISALLOW_COPY_AND_ASSIGN(CastAudioManagerFactory);
 };
 
 }  // namespace media
 }  // namespace chromecast
 
 #endif  // CHROMECAST_MEDIA_AUDIO_CAST_AUDIO_MANAGER_FACTORY_H_