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 de982977ca54c103a0d136911f1b4cfd1e83b0ce..1ee73317c8b8f4b7f84810b90b90959e09826186 100644
--- a/webrtc/voice_engine/channel_proxy.cc
+++ b/webrtc/voice_engine/channel_proxy.cc
@@ -391,6 +391,11 @@ void ChannelProxy::RegisterLegacyReceiveCodecs() {
   channel()->RegisterLegacyReceiveCodecs();
 }
 
+const std::vector<RtpContributingSource*>&
+ChannelProxy::GetContributingSources() {
+  return channel()->GetContributingSources();
+}
+
 Channel* ChannelProxy::channel() const {
   RTC_DCHECK(channel_owner_.channel());
   return channel_owner_.channel();