Implement new slider control and disable the volume control when audio is muted.

ash/system/audio/audio_observer.h

 // Copyright (c) 2012 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 "ash/ash_export.h"
 
 namespace ash {
 
 class ASH_EXPORT AudioObserver {
  public:
   virtual ~AudioObserver() {}
 
   virtual void OnVolumeChanged(float percent) = 0;
+  virtual void OnMuteToggled() = 0;
 };
 
 }  // namespace ash
 
 #endif  //ASH_SYSTEM_AUDIO_AUDIO_OBSERVER_H_