Add a NeededFrequency() method to the AudioMixer::Source interface.

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 ba58502e956c4e3105545b9d2bfd939862f42ecf..982cf1db7f0de358e3e5ae8a3b9794f1ce0f95be 100644
--- a/webrtc/voice_engine/channel_proxy.cc
+++ b/webrtc/voice_engine/channel_proxy.cc
@@ -221,6 +221,10 @@ AudioMixer::Source::AudioFrameInfo ChannelProxy::GetAudioFrameWithInfo(
   return channel()->GetAudioFrameWithInfo(sample_rate_hz, audio_frame);
 }
 
+int ChannelProxy::NeededFrequency() const {
+  return static_cast<int>(channel()->NeededFrequency(-1));
+}
+
 Channel* ChannelProxy::channel() const {
   RTC_DCHECK(channel_owner_.channel());
   return channel_owner_.channel();