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

webrtc/sdk/objc/Framework/Headers/WebRTC/RTCAudioSource.h

 /*
  *  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.
  */
 
 #import <Foundation/Foundation.h>
 
 #import <WebRTC/RTCMacros.h>
 #import <WebRTC/RTCMediaSource.h>
 
 NS_ASSUME_NONNULL_BEGIN
 
 RTC_EXPORT
 @interface RTCAudioSource : RTCMediaSource
 
 - (instancetype)init NS_UNAVAILABLE;
 
+// Sets the volume for the RTCMediaSource. |volume] is a gain value in the range
+// [0, 10].
+// Temporary fix to be able to modify volume of remote audio tracks
+// TODO: property stays here temporarily until a proper volume-api is avaialble
+// on the surface exposed by webrtc
+@property(nonatomic, assign) double volume;
+
 @end
 
 NS_ASSUME_NONNULL_END