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

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

Index: webrtc/sdk/objc/Framework/Classes/RTCAudioSource.mm
diff --git a/webrtc/sdk/objc/Framework/Classes/RTCAudioSource.mm b/webrtc/sdk/objc/Framework/Classes/RTCAudioSource.mm
index 897565244044e41433b4c05b053fb90d7fc87289..2b9c6b92d27e218459201e81ce8e4e3cdc2c99a2 100644
--- a/webrtc/sdk/objc/Framework/Classes/RTCAudioSource.mm
+++ b/webrtc/sdk/objc/Framework/Classes/RTCAudioSource.mm
@@ -16,6 +16,8 @@ @implementation RTCAudioSource {
   rtc::scoped_refptr<webrtc::AudioSourceInterface> _nativeAudioSource;
 }
 
+@synthesize volume = _volume;
+
 - (instancetype)initWithNativeAudioSource:
     (rtc::scoped_refptr<webrtc::AudioSourceInterface>)nativeAudioSource {
   RTC_DCHECK(nativeAudioSource);
@@ -38,6 +40,11 @@ - (NSString *)description {
   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 {