Implemented the GetSources() in native code.

webrtc/pc/rtpreceiver.cc

Index: webrtc/pc/rtpreceiver.cc
diff --git a/webrtc/pc/rtpreceiver.cc b/webrtc/pc/rtpreceiver.cc
index 6073b15a9739872fa89fbdb212722d35238e6519..10f948c24f92d55a8e9c855b072a6ca9de4f7460 100644
--- a/webrtc/pc/rtpreceiver.cc
+++ b/webrtc/pc/rtpreceiver.cc
@@ -97,6 +97,10 @@ void AudioRtpReceiver::Stop() {
   stopped_ = true;
 }
 
+std::vector<RtpContributingSource> AudioRtpReceiver::GetContributingSources() {
+  return channel_->GetContributingSources(ssrc_);
+}
+
 void AudioRtpReceiver::Reconfigure() {
   RTC_DCHECK(!stopped_);
   if (!channel_) {