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

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 12b8b3f7ad568582c1fb45b912c328b7b74b2deb..657a7608ec935db8514eaa33da890fa97f54d58e 100644
--- a/webrtc/audio/audio_receive_stream.cc
+++ b/webrtc/audio/audio_receive_stream.cc
@@ -278,7 +278,11 @@ AudioMixer::Source::AudioFrameInfo AudioReceiveStream::GetAudioFrameWithInfo(
   return channel_proxy_->GetAudioFrameWithInfo(sample_rate_hz, audio_frame);
 }
 
-int AudioReceiveStream::Ssrc() {
+int AudioReceiveStream::PreferredSampleRate() const {
+  return channel_proxy_->NeededFrequency();
+}
+
+int AudioReceiveStream::Ssrc() const {
   return config_.rtp.local_ssrc;
 }