RTCRtpReceiver.getContributingSources() added.

chrome/test/data/webrtc/peerconnection_rtp.js

Index: chrome/test/data/webrtc/peerconnection_rtp.js
diff --git a/chrome/test/data/webrtc/peerconnection_rtp.js b/chrome/test/data/webrtc/peerconnection_rtp.js
index efffd8cce07203e3a09b86f19a7d17a5cb7dd55d..755c0972ad92fd4950f3c52b71e92a59a3065325 100644
--- a/chrome/test/data/webrtc/peerconnection_rtp.js
+++ b/chrome/test/data/webrtc/peerconnection_rtp.js
@@ -64,6 +64,8 @@ function verifyRtpReceivers(expectedNumTracks = null) {
       throw failTest('receiver is null or undefined.');
     if (receiver.track == null)
       throw failTest('receiver.track is null or undefined.');
+    if (receiver.getContributingSources().length != 0)
+      throw failTest('receiver.getContributingSources() is not empty.');
     receivers.add(receiver);
     receiverTracks.add(receiver.track);
   });