Implemented the GetSources() in native code.

webrtc/media/engine/webrtcvoiceengine.cc

Index: webrtc/media/engine/webrtcvoiceengine.cc
diff --git a/webrtc/media/engine/webrtcvoiceengine.cc b/webrtc/media/engine/webrtcvoiceengine.cc
index 8fcda89962ee768153b9d6337a862309b7e67aab..bff27344a3768ad3606275b59767d65492ab2da9 100644
--- a/webrtc/media/engine/webrtcvoiceengine.cc
+++ b/webrtc/media/engine/webrtcvoiceengine.cc
@@ -1581,6 +1581,12 @@ class WebRtcVoiceMediaChannel::WebRtcAudioReceiveStream {
     playout_ = playout;
   }
 
+  const std::vector<webrtc::RtpContributingSource>& GetContributingSources() {
+    RTC_DCHECK(worker_thread_checker_.CalledOnValidThread());
+    RTC_DCHECK(stream_);
+    return stream_->GetContributingSources();
+  }
+
  private:
   void RecreateAudioReceiveStream() {
     RTC_DCHECK(worker_thread_checker_.CalledOnValidThread());
@@ -2600,6 +2606,15 @@ void WebRtcVoiceMediaChannel::SetRawAudioSink(
   it->second->SetRawAudioSink(std::move(sink));
 }
 
+const std::vector<webrtc::RtpContributingSource>&
+WebRtcVoiceMediaChannel::GetContributingSources(uint32_t ssrc) {
+  auto it = recv_streams_.find(ssrc);
+  RTC_DCHECK(it != recv_streams_.end())
+      << "Attempting to get contributing sources for SSRC:" << ssrc
+      << " which doesn't exist.";
+  return it->second->GetContributingSources();
+}
+
 int WebRtcVoiceMediaChannel::GetReceiveChannelId(uint32_t ssrc) const {
   RTC_DCHECK(worker_thread_checker_.CalledOnValidThread());
   const auto it = recv_streams_.find(ssrc);