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

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

Index: webrtc/sdk/objc/Framework/Headers/WebRTC/RTCAudioSource.h
diff --git a/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCAudioSource.h b/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCAudioSource.h
index a86f42bbb75cd1f0d7d62806cac1335c1d62ee09..682f9cc364d1d3379e2fb93f8146c7b4507a47b7 100644
--- a/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCAudioSource.h
+++ b/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCAudioSource.h
@@ -20,6 +20,12 @@ RTC_EXPORT
 
 - (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