Add a new audio extension event OnLevelChanged.

ash/system/audio/audio_observer.h

 // Copyright 2014 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 ASH_SYSTEM_AUDIO_AUDIO_OBSERVER_H_
 #define ASH_SYSTEM_AUDIO_AUDIO_OBSERVER_H_
 
+#include "base/basictypes.h"
+
 namespace ash {
 
 class AudioObserver {
  public:
   virtual ~AudioObserver() {}
 
   // Called when output volume changed.
-  virtual void OnOutputVolumeChanged() = 0;
+  virtual void OnOutputNodeVolumeChanged(uint64 node_id, double volume) = 0;
 
   // Called when output mute state changed.
   virtual void OnOutputMuteChanged() = 0;
 
   // Called when audio nodes changed.
   virtual void OnAudioNodesChanged() = 0;
 
   // Called when active audio output node changed.
   virtual void OnActiveOutputNodeChanged() = 0;
 
   // Called when active audio input node changed.
   virtual void OnActiveInputNodeChanged() = 0;
 };
 
 }  // namespace ash
 
 #endif  // ASH_SYSTEM_AUDIO_AUDIO_OBSERVER_H_