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

webrtc/media/engine/fakewebrtccall.cc

Index: webrtc/media/engine/fakewebrtccall.cc
diff --git a/webrtc/media/engine/fakewebrtccall.cc b/webrtc/media/engine/fakewebrtccall.cc
index fd020d43bd72d457ced8de8d6816ce4607abdaae..914a403f41e0e3045610a9b3e502cdb7e13ce2ab 100644
--- a/webrtc/media/engine/fakewebrtccall.cc
+++ b/webrtc/media/engine/fakewebrtccall.cc
@@ -57,7 +57,7 @@ webrtc::AudioSendStream::Stats FakeAudioSendStream::GetStats() const {
 
 FakeAudioReceiveStream::FakeAudioReceiveStream(
     const webrtc::AudioReceiveStream::Config& config)
-    : config_(config), received_packets_(0) {
+    : config_(config) {
   RTC_DCHECK(config.voe_channel_id != -1);
 }
 
@@ -93,6 +93,10 @@ void FakeAudioReceiveStream::SetSink(
   sink_ = std::move(sink);
 }
 
+void FakeAudioReceiveStream::SetGain(float gain) {
+  gain_ = gain;
+}
+
 FakeVideoSendStream::FakeVideoSendStream(
     const webrtc::VideoSendStream::Config& config,
     const webrtc::VideoEncoderConfig& encoder_config)