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..24e2fb9d5ac8d90c106cbaff3349d5e5230d8702 100644
--- a/extensions/browser/api/audio/audio_service.h
+++ b/extensions/browser/api/audio/audio_service.h
@@ -17,6 +17,7 @@ namespace extensions {
 
 using OutputInfo = std::vector<linked_ptr<core_api::audio::OutputDeviceInfo>>;
 using InputInfo = std::vector<linked_ptr<core_api::audio::InputDeviceInfo>>;
+using DevicesInfo = std::vector<linked_ptr<core_api::audio::AudioDeviceInfo>>;
 using DeviceIdList = std::vector<std::string>;
 
 class AudioService {
@@ -26,6 +27,10 @@ class AudioService {
     // Called when anything changes to the audio device configuration.
     virtual void OnDeviceChanged() = 0;
 
+    // Called when the volume of an active output node changes.
+    virtual void OnOutputNodeVolumeChanged(const std::string& id,
+                                           double volume) = 0;
+
    protected:
     virtual ~Observer() {}
   };
@@ -34,6 +39,9 @@ class AudioService {
   typedef base::Callback<void(const OutputInfo&, const InputInfo&, bool)>
       GetInfoCallback;
 
+  // Callback type for completing to get audio devices information.
+  typedef base::Callback<void(const DevicesInfo&, bool)> GetDeviceInfoCallback;
+
   // Creates a platform-specific AudioService instance.
   static AudioService* CreateInstance();
 
@@ -63,6 +71,10 @@ class AudioService {
                                    int volume,
                                    int gain) = 0;
 
+  // Starts to query audio devices information. Should be called on UI thread.
+  // The |callback| will be invoked once the query is completed.
+  virtual void StartGetDeviceInfo(const GetDeviceInfoCallback& callback) = 0;
+
  protected:
   AudioService() {}