Implemented the GetSources() in native code.

webrtc/modules/rtp_rtcp/include/rtp_receiver.h

Index: webrtc/modules/rtp_rtcp/include/rtp_receiver.h
diff --git a/webrtc/modules/rtp_rtcp/include/rtp_receiver.h b/webrtc/modules/rtp_rtcp/include/rtp_receiver.h
index 54a99c93a1de5fc4dbb62a08536d529242ca9572..462458ddb1defc7aa5569fcfe0b343ec7004a869 100644
--- a/webrtc/modules/rtp_rtcp/include/rtp_receiver.h
+++ b/webrtc/modules/rtp_rtcp/include/rtp_receiver.h
@@ -11,6 +11,9 @@
 #ifndef WEBRTC_MODULES_RTP_RTCP_INCLUDE_RTP_RECEIVER_H_
 #define WEBRTC_MODULES_RTP_RTCP_INCLUDE_RTP_RECEIVER_H_
 
+#include <vector>
+
+#include "webrtc/api/rtpreceiverinterface.h"
 #include "webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h"
 #include "webrtc/typedefs.h"
 
@@ -20,6 +23,28 @@ struct CodecInst;
 class RTPPayloadRegistry;
 class VideoCodec;
 
+class RtpContributingSource : public RtpContributingSourceInterface {
+ public:
+  RtpContributingSource() : timestamp_(0), source_(0) {}
+
+  explicit RtpContributingSource(int64_t timestamp, uint32_t source)
+      : timestamp_(timestamp), source_(source) {}
+
+  int64_t timestamp() const override { return timestamp_; }
+
+  uint32_t source() const override { return source_; }
+
+  // Always return 0 in the current implementation.
+  // TODO(zhihuang): Implement this method to return real audio level.
+  int8_t audio_level() const override { return 0; }

[hbos, 2017/03/30 09:51:54]

Return a value that doesn't map to a meaningful value, say -1 or
rtc::Optional<>()

[Zhi Huang, 2017/03/31 06:44:04]

Done.

+
+  void set_timestamp(int64_t timestamp) { timestamp_ = timestamp; }
+
+ private:
+  int64_t timestamp_;
+  uint32_t source_;
+};
+
 class TelephoneEventHandler {
  public:
   virtual ~TelephoneEventHandler() {}
@@ -89,6 +114,9 @@ class RtpReceiver {
 
   // Returns the current energy of the RTP stream received.
   virtual int32_t Energy(uint8_t array_of_energy[kRtpCsrcSize]) const = 0;
+
+  virtual const std::vector<RtpContributingSource*>&
+  GetContributingSources() = 0;
 };
 }  // namespace webrtc