cras_audio_handler add SetOutputMono method

chromeos/audio/cras_audio_handler.h

Index: chromeos/audio/cras_audio_handler.h
diff --git a/chromeos/audio/cras_audio_handler.h b/chromeos/audio/cras_audio_handler.h
index 4d98b60b192c544936ad08fa300c0c10f8576e6e..13c2848735e76e93971113844ca3fe8552968fc7 100644
--- a/chromeos/audio/cras_audio_handler.h
+++ b/chromeos/audio/cras_audio_handler.h
@@ -66,6 +66,9 @@ class CHROMEOS_EXPORT CrasAudioHandler : public CrasAudioClient::Observer,
     // Called when active audio input node changed.
     virtual void OnActiveInputNodeChanged();
 
+    // Called when output channel remixing changed.
+    virtual void OnOuputChannelRemixingChanged(bool mono_on);
+
    protected:
     AudioObserver();
     virtual ~AudioObserver();
@@ -206,6 +209,12 @@ class CHROMEOS_EXPORT CrasAudioHandler : public CrasAudioClient::Observer,
   // If the feature is not supported on the device, nothing happens.
   virtual void SwapInternalSpeakerLeftRightChannel(bool swap);
 
+  // Accessibility audio setting: sets the output mono or not.
+  virtual void SetOutputMono(bool mono_on);
+
+  // Returns true if output mono is enabled.
+  virtual bool IsOutputMonoEnabled() const;
+
   // Enables error logging.
   virtual void LogErrors();
 
@@ -408,6 +417,10 @@ class CHROMEOS_EXPORT CrasAudioHandler : public CrasAudioClient::Observer,
 
   bool output_mute_locked_;
 
+  // Audio output channel counts.
+  int32_t channels_;

[jennyz, 2016/03/30 23:39:26]

How about naming it output_channels_? 

+  bool output_mono_on_;
+
   // Failures are not logged at startup, since CRAS may not be running yet.
   bool log_errors_;