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}

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 {