Implemented the GetSources() in native code.

webrtc/voice_engine/channel_proxy.cc

Index: webrtc/voice_engine/channel_proxy.cc
diff --git a/webrtc/voice_engine/channel_proxy.cc b/webrtc/voice_engine/channel_proxy.cc
index bb56b078d193aedb8859e2bda89d487fc972cbe1..63c0678b8ae91ca61009662162babac233bae8cf 100644
--- a/webrtc/voice_engine/channel_proxy.cc
+++ b/webrtc/voice_engine/channel_proxy.cc
@@ -396,6 +396,11 @@ void ChannelProxy::RegisterLegacyReceiveCodecs() {
   channel()->RegisterLegacyReceiveCodecs();
 }
 
+std::vector<RtpContributingSource> ChannelProxy::GetContributingSources() {
+  RTC_DCHECK(worker_thread_checker_.CalledOnValidThread());
+  return channel()->GetContributingSources();
+}
+
 Channel* ChannelProxy::channel() const {
   RTC_DCHECK(channel_owner_.channel());
   return channel_owner_.channel();