Add a new audio extension event OnLevelChanged.

ash/system/audio/audio_observer.h

Index: ash/system/audio/audio_observer.h
diff --git a/ash/system/audio/audio_observer.h b/ash/system/audio/audio_observer.h
index 602322678f73db59bd5c6341460a730d581a7e41..07a8024671c190760476f31f2c043d07cd8b96f3 100644
--- a/ash/system/audio/audio_observer.h
+++ b/ash/system/audio/audio_observer.h
@@ -5,6 +5,8 @@
 #ifndef ASH_SYSTEM_AUDIO_AUDIO_OBSERVER_H_
 #define ASH_SYSTEM_AUDIO_AUDIO_OBSERVER_H_
 
+#include "base/basictypes.h"
+
 namespace ash {
 
 class AudioObserver {
@@ -12,7 +14,7 @@ class AudioObserver {
   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;