Reland of Implemented the GetSources() in native code.

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 fbd61ccfd4791e3386a9cd68565c053782fe2616..cb90a68a0f72e6898fe11726d31d2b99c9770c98 100644
--- a/webrtc/audio/audio_receive_stream.cc
+++ b/webrtc/audio/audio_receive_stream.cc
@@ -219,6 +219,11 @@
   channel_proxy_->SetChannelOutputVolumeScaling(gain);
 }
 
+std::vector<RtpSource> AudioReceiveStream::GetSources() const {
+  RTC_DCHECK_RUN_ON(&worker_thread_checker_);
+  return channel_proxy_->GetSources();
+}
+
 AudioMixer::Source::AudioFrameInfo AudioReceiveStream::GetAudioFrameWithInfo(
     int sample_rate_hz,
     AudioFrame* audio_frame) {