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

webrtc/audio/audio_receive_stream.cc

Index: webrtc/audio/audio_receive_stream.cc
diff --git a/webrtc/audio/audio_receive_stream.cc b/webrtc/audio/audio_receive_stream.cc
index 15e4f6107a62bcf0c0b66e9c8c2b31a3128251cb..c933f78ea2b9c6d8574261755980b4f5302bf6ae 100644
--- a/webrtc/audio/audio_receive_stream.cc
+++ b/webrtc/audio/audio_receive_stream.cc
@@ -210,6 +210,11 @@ void AudioReceiveStream::SetSink(std::unique_ptr<AudioSinkInterface> sink) {
   channel_proxy_->SetSink(std::move(sink));
 }
 
+void AudioReceiveStream::SetGain(float gain) {
+  RTC_DCHECK(thread_checker_.CalledOnValidThread());
+  channel_proxy_->SetChannelOutputVolumeScaling(gain);
+}
+
 const webrtc::AudioReceiveStream::Config& AudioReceiveStream::config() const {
   RTC_DCHECK(thread_checker_.CalledOnValidThread());
   return config_;