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..3ce859e4b38057efd24f544027b227bffcfe4d70 100644
--- a/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCAudioSource.h
+++ b/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