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

This is a short term solution to change the volume of a RTCAudioTrack (which contains an RTCAudioSource property) until applyConstraints for RTCMediaStreamTracks has been implemented.
This CL adds one new Objective-C method to AudioSourceInterface's wrapper: -(void)setVolume:(double)volume

BUG=webrtc:6533, webrtc:6805

This is my first CL for Chromium/WebRTC, so please let me know if I did something wrong.

Review-Url: https://codereview.webrtc.org/2534843002
Cr-Commit-Position: refs/heads/master@{#16809}

AUTHORS

@@ -1,4 +1,4 @@
-# Names should be added to this file like so:
+# Names should be added to this file like so:
 # Name or Organization <email address>
 
 Adam Fedor <adam.fedor@gmail.com>
@@ -13,9 +13,9 @@ Christophe Dumez <ch.dumez@samsung.com>
 Cody Barnes <conceptgenesis@gmail.com>
 Colin Plumb
 Eric Rescorla, RTFM Inc. <ekr@rtfm.com>
+Frederik Riedel, Frogg GmbH <frederik.riedel@frogg.io>
 Giji Gangadharan <giji.g@samsung.com>
 Graham Yoakum <gyoakum@skobalt.com>
-Hugues Ekra <hekra01@gmail.com>
 Jake Hilton <jakehilton@gmail.com>
 James H. Brown <jbrown@burgoyne.com>
 Jiawei Ou <jiawei.ou@gmail.com>

webrtc/sdk/objc/Framework/Classes/RTCAudioSource.mm

@@ -16,6 +16,8 @@
   rtc::scoped_refptr<webrtc::AudioSourceInterface> _nativeAudioSource;
 }
 
+@synthesize volume = _volume;
+
 - (instancetype)initWithNativeAudioSource:
     (rtc::scoped_refptr<webrtc::AudioSourceInterface>)nativeAudioSource {
   RTC_DCHECK(nativeAudioSource);
@@ -38,6 +40,11 @@
   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 {

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

@@ -20,6 +20,13 @@ RTC_EXPORT
 
 - (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