Add a new audio extension event OnLevelChanged.

extensions/browser/api/audio/audio_service.h

Index: extensions/browser/api/audio/audio_service.h
diff --git a/extensions/browser/api/audio/audio_service.h b/extensions/browser/api/audio/audio_service.h
index 6d985ebd23b1f8f660897a1dfdb0228be9f34aaf..5d0ac0708be2a275d19419539cbdd9160f56e381 100644
--- a/extensions/browser/api/audio/audio_service.h
+++ b/extensions/browser/api/audio/audio_service.h
@@ -26,6 +26,9 @@ class AudioService {
     // Called when anything changes to the audio device configuration.
     virtual void OnDeviceChanged() = 0;
 
+    // Called when the sound level of an active audio node changes.
+    virtual void OnLevelChanged(const std::string& id, int level) = 0;
+
    protected:
     virtual ~Observer() {}
   };