Implemented the GetSources() in native code.

webrtc/pc/channel.cc

Index: webrtc/pc/channel.cc
diff --git a/webrtc/pc/channel.cc b/webrtc/pc/channel.cc
index ecc1c9a633c6d4a935a146f048483256ccdc5d49..fd44a04141a93d59d1591e4f3c03c92c3ba0ffc8 100644
--- a/webrtc/pc/channel.cc
+++ b/webrtc/pc/channel.cc
@@ -23,6 +23,7 @@
 #include "webrtc/base/trace_event.h"
 #include "webrtc/media/base/mediaconstants.h"
 #include "webrtc/media/base/rtputils.h"
+#include "webrtc/media/engine/webrtcvoiceengine.h"
 #include "webrtc/p2p/base/packettransportinternal.h"
 #include "webrtc/pc/channelmanager.h"
 
@@ -1663,6 +1664,13 @@ bool VoiceChannel::GetStats(VoiceMediaInfo* stats) {
                                             media_channel(), stats));
 }
 
+std::vector<webrtc::RtpSource> VoiceChannel::GetSources(uint32_t ssrc) const {
+  return worker_thread()->Invoke<std::vector<webrtc::RtpSource>>(
+      RTC_FROM_HERE,
+      Bind(&WebRtcVoiceMediaChannel::GetSources,
+           static_cast<WebRtcVoiceMediaChannel*>(media_channel()), ssrc));
+}
+
 void VoiceChannel::StartMediaMonitor(int cms) {
   media_monitor_.reset(new VoiceMediaMonitor(media_channel(), worker_thread(),
       rtc::Thread::Current()));