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

webrtc/voice_engine/channel_proxy.cc

Index: webrtc/voice_engine/channel_proxy.cc
diff --git a/webrtc/voice_engine/channel_proxy.cc b/webrtc/voice_engine/channel_proxy.cc
index d16167f9bb15bfff51e0be616531fd938a1c930c..2d4fe556bef3e79e2b6681249307ff4a50d909b2 100644
--- a/webrtc/voice_engine/channel_proxy.cc
+++ b/webrtc/voice_engine/channel_proxy.cc
@@ -182,10 +182,17 @@ bool ChannelProxy::ReceivedRTCPPacket(const uint8_t* packet, size_t length) {
 }
 
 const rtc::scoped_refptr<AudioDecoderFactory>&
-ChannelProxy::GetAudioDecoderFactory() const {
+    ChannelProxy::GetAudioDecoderFactory() const {
+  RTC_DCHECK(thread_checker_.CalledOnValidThread());
   return channel()->GetAudioDecoderFactory();
 }
 
+void ChannelProxy::SetChannelOutputVolumeScaling(float scaling) {
+  RTC_DCHECK(thread_checker_.CalledOnValidThread());
+  int error = channel()->SetChannelOutputVolumeScaling(scaling);
+  RTC_DCHECK_EQ(0, error);
+}
+
 Channel* ChannelProxy::channel() const {
   RTC_DCHECK(channel_owner_.channel());
   return channel_owner_.channel();