[chromecast] Simplify CastAudioOutputStream.

https://codereview.chromium.org/1908423006/ makes CastAudioManager
run on CMA thread. Now there is no need to post tasks when using
CMA backend in CastAudioAoutputStream.

BUG=594234

Review URL: https://codereview.chromium.org/1913973003

Cr-Commit-Position: refs/heads/master@{#389813}

chromecast/media/audio/cast_audio_manager.h

@@ -35,8 +35,7 @@ class CastAudioManager : public ::media::AudioManagerBase {
   ::media::AudioParameters GetInputStreamParameters(
       const std::string& device_id) override;
 
-  // This must be called on cast media thread.
-  // See chromecast::media::MediaMessageLoop.
+  // This must be called on audio thread.
   virtual std::unique_ptr<MediaPipelineBackend> CreateMediaPipelineBackend(
       const MediaPipelineDeviceParams& params);
 

chromecast/media/audio/cast_audio_output_stream.h

@@ -11,10 +11,6 @@
 #include "media/audio/audio_io.h"
 #include "media/base/audio_parameters.h"
 
-namespace base {
-class SingleThreadTaskRunner;
-}  // namespace base
-
 namespace chromecast {
 namespace media {
 
@@ -38,7 +34,6 @@ class CastAudioOutputStream : public ::media::AudioOutputStream {
  private:
   class Backend;
 
-  void OnClosed();
   void PushBuffer();
   void OnPushBufferComplete(bool success);
 
@@ -54,9 +49,6 @@ class CastAudioOutputStream : public ::media::AudioOutputStream {
   bool push_in_progress_;
   base::TimeTicks next_push_time_;
 
-  scoped_refptr<base::SingleThreadTaskRunner> audio_task_runner_;
-  scoped_refptr<base::SingleThreadTaskRunner> backend_task_runner_;
-
   base::WeakPtrFactory<CastAudioOutputStream> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(CastAudioOutputStream);