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 c6c9b30b2a29e294de3f63b8028d5292762e658f..2cb9801c44ed4b560ee9fe9a11c11595f3700968 100644
--- a/webrtc/audio/audio_receive_stream.cc
+++ b/webrtc/audio/audio_receive_stream.cc
@@ -205,6 +205,11 @@ 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_;