Implemented the GetSources() in native code.

webrtc/pc/peerconnection_integrationtest.cc

 /*
  *  Copyright 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 2737 matching lines @@
   caller()->CreateAndSetAndSignalOffer();
   ASSERT_TRUE_WAIT(SignalingStateStable(), kDefaultTimeout);
 
   // Verify frames are still received end-to-end.
   ExpectNewFramesReceivedWithWait(
       kDefaultExpectedAudioFrameCount, kDefaultExpectedVideoFrameCount,
       kDefaultExpectedAudioFrameCount, kDefaultExpectedVideoFrameCount,
       kMaxWaitForFramesMs);
 }
 
+TEST_F(PeerConnectionIntegrationTest, GetSources) {
+  ASSERT_TRUE(CreatePeerConnectionWrappers());
+  ConnectFakeSignaling();
+  caller()->AddAudioVideoMediaStream();
+  callee()->AddAudioVideoMediaStream();
+  caller()->CreateAndSetAndSignalOffer();
+  ASSERT_TRUE_WAIT(SignalingStateStable(), kDefaultTimeout);
+  ExpectNewFramesReceivedWithWait(
+      kDefaultExpectedAudioFrameCount, kDefaultExpectedVideoFrameCount,
+      kDefaultExpectedAudioFrameCount, kDefaultExpectedVideoFrameCount,

[Taylor Brandstetter, 2017/04/05 04:27:55]

nit: Only really need one audio track, from caller to callee, and only need to
wait for one audio frame to be received.

[Zhi Huang, 2017/04/06 03:09:50]

I thought if it works in more complicated cases, it will also works in the one
way audio only case. But I guess the simple case would be enough since we are
only testing the pipeline.

+      kMaxWaitForFramesMs);
+  ASSERT_GT(callee()->pc()->GetReceivers().size(), 0u);
+  auto receiver = callee()->pc()->GetReceivers()[0];
+  ASSERT_EQ(receiver->media_type(), cricket::MEDIA_TYPE_AUDIO);
+
+  auto contributing_sources = receiver->GetSources();
+  ASSERT_GT(receiver->GetParameters().encodings.size(), 0u);
+  EXPECT_EQ(receiver->GetParameters().encodings[0].ssrc,
+            contributing_sources[0].source_id());
+}
+
 }  // namespace
 
 #endif  // if !defined(THREAD_SANITIZER)