Added support for changing the volume of RTCAudioSource as discussed in BUG=webrtc:6533

webrtc/sdk/objc/Framework/Classes/RTCAudioSource.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 20 matching lines @@
                                      type:(RTCMediaSourceType)type {
   RTC_NOTREACHED();
   return nil;
 }
 
 - (NSString *)description {
   NSString *stateString = [[self class] stringForState:self.state];
   return [NSString stringWithFormat:@"RTCAudioSource( %p ): %@", self, stateString];
 }
 
+- (void)setVolume:(double)volume {
+  _volume = volume;
+  _nativeAudioSource->SetVolume(volume);
+}
+
 #pragma mark - Private
 
 - (rtc::scoped_refptr<webrtc::AudioSourceInterface>)nativeAudioSource {
   return _nativeAudioSource;
 }
 
 @end