Add observer for AVAudioSession.outputVolume

webrtc/modules/audio_device/ios/objc/RTCAudioSessionDelegateAdapter.mm

 /*
  *  Copyright 2016 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 63 matching lines @@
     didChangeCanPlayOrRecord:(BOOL)canPlayOrRecord {
   _observer->OnCanPlayOrRecordChange(canPlayOrRecord);
 }
 
 - (void)audioSessionDidStartPlayOrRecord:(RTCAudioSession *)session {
 }
 
 - (void)audioSessionDidStopPlayOrRecord:(RTCAudioSession *)session {
 }
 
+- (void)audioVolumeDidChange:(float)volume {

[tkchin_webrtc, 2017/05/25 20:24:33]

not needed? (since it's optional)

+}
+
 @end