Add a new audio extension event OnLevelChanged.

ash/system/audio/tray_audio.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_TRAY_AUDIO_H_
 #define ASH_SYSTEM_AUDIO_TRAY_AUDIO_H_
 
 #include "ash/system/audio/audio_observer.h"
 #include "ash/system/tray/tray_image_item.h"
 #include "base/memory/scoped_ptr.h"
@@ skipping 35 matching lines @@
 
   // Overridden from SystemTrayItem.
   views::View* CreateDefaultView(user::LoginStatus status) override;
   views::View* CreateDetailedView(user::LoginStatus status) override;
   void DestroyDefaultView() override;
   void DestroyDetailedView() override;
   bool ShouldHideArrow() const override;
   bool ShouldShowShelf() const override;
 
   // Overridden from AudioObserver.
-  void OnOutputVolumeChanged() override;
+  void OnOutputNodeVolumeChanged(uint64 node_id, double volume) override;
   void OnOutputMuteChanged() override;
   void OnAudioNodesChanged() override;
   void OnActiveOutputNodeChanged() override;
   void OnActiveInputNodeChanged() override;
 
   // Overridden from gfx::DisplayObserver.
   void OnDisplayAdded(const gfx::Display& new_display) override;
   void OnDisplayRemoved(const gfx::Display& old_display) override;
   void OnDisplayMetricsChanged(const gfx::Display& display,
                                uint32_t changed_metrics) override;
 
   void ChangeInternalSpeakerChannelMode();
 
   DISALLOW_COPY_AND_ASSIGN(TrayAudio);
 };
 
 }  // namespace ash
 
 #endif  // ASH_SYSTEM_AUDIO_TRAY_AUDIO_H_