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..7c39403dec0d2132058412791284c571a92339d2 100644
--- a/webrtc/pc/rtpreceiver.cc
+++ b/webrtc/pc/rtpreceiver.cc
@@ -12,7 +12,9 @@
 
 #include "webrtc/api/mediastreamtrackproxy.h"
 #include "webrtc/api/videosourceproxy.h"
+#include "webrtc/base/ptr_util.h"
 #include "webrtc/base/trace_event.h"
+#include "webrtc/modules/rtp_rtcp/include/rtp_receiver.h"
 #include "webrtc/pc/audiotrack.h"
 #include "webrtc/pc/videotrack.h"
 
@@ -97,6 +99,10 @@ void AudioRtpReceiver::Stop() {
   stopped_ = true;
 }
 
+std::vector<RtpContributingSource> AudioRtpReceiver::GetContributingSources() {
+  return channel_->GetContributingSources(ssrc_);
+}
+
 void AudioRtpReceiver::Reconfigure() {
   RTC_DCHECK(!stopped_);
   if (!channel_) {