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 to the RTCMediaSource. |volume| is in  the range of [0, 10].

[the sun, 2016/12/01 18:24:19]

Make that read:

"Sets the volume for the RTCMediaSource. |volume] is a gain value in the range
[0, 10].

[Frederik, 2016/12/01 18:54:31]

Done.

+// Temporary fix to be able to modify volume of remote audio tracks
+// TODO: implementing support for applyConstraints() to MediaStreamTrack will

[the sun, 2016/11/30 11:38:04]

TODO should have an owner

+// replace this implementation

[hta-webrtc, 2016/12/01 13:59:15]

I think the surface exposed by webrtc should have a volume property and/or a
setvolume method. The constraints concept needs to live in Chromium alone. But
(0..10) is an odd value range, and the nature of the relationship to the volume
is not specified, so the API needs to be temporary. So leaving a TODO is nice.

(ref discussion with @thesun.

[the sun, 2016/12/01 18:24:19]

The [0..10] range stems from a DCHECK in RemoteAudioSource which appears
arbitrary but not unreasonable. Lower layers accept any gain value and a gain of
10 gives a 20dB boost, which should be enough for many uses. For a temp API I
think we can leave it like that.

+@property(nonatomic, assign) double volume;
+
 @end
 
 NS_ASSUME_NONNULL_END