Change the default rtcp mux policy from negotiate to require.

third_party/WebKit/Source/modules/peerconnection/RTCPeerConnection.cpp

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 245 matching lines @@
   if (bundlePolicyString == "max-compat") {
     bundlePolicy = WebRTCBundlePolicy::kMaxCompat;
   } else if (bundlePolicyString == "max-bundle") {
     bundlePolicy = WebRTCBundlePolicy::kMaxBundle;
   } else {
     DCHECK_EQ(bundlePolicyString, "balanced");
   }
 
   // For the histogram value of "WebRTC.PeerConnection.SelectedRtcpMuxPolicy".
   *selectedRtcpMuxPolicy = RtcpMuxPolicyDefault;
-  WebRTCRtcpMuxPolicy rtcpMuxPolicy = WebRTCRtcpMuxPolicy::kNegotiate;
+  WebRTCRtcpMuxPolicy rtcpMuxPolicy = WebRTCRtcpMuxPolicy::kRequire;
   if (configuration.hasRtcpMuxPolicy()) {
     String rtcpMuxPolicyString = configuration.rtcpMuxPolicy();
     if (rtcpMuxPolicyString == "require") {
       *selectedRtcpMuxPolicy = RtcpMuxPolicyRequire;
       rtcpMuxPolicy = WebRTCRtcpMuxPolicy::kRequire;

[honghaiz3, 2016/11/22 00:19:35]

I think you need to move this line to the else clause and change it to 
rtcpMuxPolicy = WebRTCRtcpMuxPolicy::kNegotiate; 

[zhihuang1, 2016/11/22 00:31:55]

Ah, you are right. This is a silly mistake.

     } else {
       DCHECK_EQ(rtcpMuxPolicyString, "negotiate");
       *selectedRtcpMuxPolicy = RtcpMuxPolicyNegotiate;
     }
   }
 
   WebRTCConfiguration webConfiguration;
   webConfiguration.iceTransportPolicy = iceTransportPolicy;
   webConfiguration.bundlePolicy = bundlePolicy;
   webConfiguration.rtcpMuxPolicy = rtcpMuxPolicy;
@@ skipping 1199 matching lines @@
 DEFINE_TRACE(RTCPeerConnection) {
   visitor->trace(m_localStreams);
   visitor->trace(m_remoteStreams);
   visitor->trace(m_dispatchScheduledEventRunner);
   visitor->trace(m_scheduledEvents);
   EventTargetWithInlineData::trace(visitor);
   ActiveDOMObject::trace(visitor);
 }
 
 }  // namespace blink