Implemented the GetSources() in native code.

webrtc/media/engine/webrtcvoiceengine.h

Index: webrtc/media/engine/webrtcvoiceengine.h
diff --git a/webrtc/media/engine/webrtcvoiceengine.h b/webrtc/media/engine/webrtcvoiceengine.h
index a27c8f3aebd3e9d15457eba03ed97e76fce0ce8d..4d71c893340ca54644ba739c047e1e629cef9a37 100644
--- a/webrtc/media/engine/webrtcvoiceengine.h
+++ b/webrtc/media/engine/webrtcvoiceengine.h
@@ -16,6 +16,7 @@
 #include <string>
 #include <vector>
 
+#include "webrtc/api/rtpreceiverinterface.h"
 #include "webrtc/base/buffer.h"
 #include "webrtc/base/constructormagic.h"
 #include "webrtc/base/networkroute.h"
@@ -210,6 +211,8 @@ class WebRtcVoiceMediaChannel final : public VoiceMediaChannel,
       uint32_t ssrc,
       std::unique_ptr<webrtc::AudioSinkInterface> sink) override;
 
+  std::vector<webrtc::RtpSource> GetSources(uint32_t ssrc) const;
+
   // implements Transport interface
   bool SendRtp(const uint8_t* data,
                size_t len,