Add a new audio extension event OnLevelChanged.

extensions/browser/api/audio/audio_service.cc

 // Copyright (c) 2013 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.
 
 #include "extensions/browser/api/audio/audio_service.h"
 
 namespace extensions {
 
 class AudioServiceImpl : public AudioService {
  public:
@@ skipping 34 matching lines @@
 void AudioServiceImpl::SetActiveDevices(const DeviceIdList& device_list) {
 }
 
 bool AudioServiceImpl::SetDeviceProperties(const std::string& device_id,
                                            bool muted,
                                            int volume,
                                            int gain) {
   return false;
 }
 
+void AudioServiceImpl::StartGetDeviceInfo(
+    const GetDeviceInfoCallback& callback) {
+  // TODO: implement this for platforms other than Chrome OS.
+  if (!callback.is_null())
+    callback.Run(DevicesInfo(), false);
+}
+
 }  // namespace extensions