Add AudioReceiveStream::SetGain() method and use that in WVoMC::SetOutputVolume().

webrtc/audio_receive_stream.h

Index: webrtc/audio_receive_stream.h
diff --git a/webrtc/audio_receive_stream.h b/webrtc/audio_receive_stream.h
index 16ed7692636a6bea53d004e7030532379864d123..e0e9536a58eac4db5dfef6e08d4100f7f9df1aa0 100644
--- a/webrtc/audio_receive_stream.h
+++ b/webrtc/audio_receive_stream.h
@@ -127,6 +127,10 @@ class AudioReceiveStream {
   // of feeding to the AEC.
   virtual void SetSink(std::unique_ptr<AudioSinkInterface> sink) = 0;
 
+  // Sets playback gain of the stream, applied when mixing, and thus after it
+  // is potentially forwarded to any attached AudioSinkInterface implementation.
+  virtual void SetGain(float gain) = 0;
+
  protected:
   virtual ~AudioReceiveStream() {}
 };