Provide better message for when RTCP mux "require" policy is triggered.

webrtc/pc/channel.cc

 /*
  *  Copyright 2004 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 1165 matching lines @@
     SafeSetError("Failed to setup SRTP filter.", error_desc);
     return false;
   }
   return true;
 }
 
 bool BaseChannel::SetRtcpMux_n(bool enable,
                                ContentAction action,
                                ContentSource src,
                                std::string* error_desc) {
+  // Provide a more specific error message for the RTCP mux "require" policy
+  // case.
+  if (rtcp_mux_required_ && !enable) {
+    SafeSetError(
+        "rtcpMuxPolicy is 'require', but media description does not "
+        "contain 'a=rtcp-mux'.",
+        error_desc);
+    return false;
+  }
   bool ret = false;
   switch (action) {
     case CA_OFFER:
       ret = rtcp_mux_filter_.SetOffer(enable, src);
       break;
     case CA_PRANSWER:
       // This may activate RTCP muxing, but we don't yet destroy the channel
       // because the final answer may deactivate it.
       ret = rtcp_mux_filter_.SetProvisionalAnswer(enable, src);
       break;
@@ skipping 1201 matching lines @@
   signaling_thread()->Post(RTC_FROM_HERE, this, MSG_READYTOSENDDATA,
                            new DataChannelReadyToSendMessageData(writable));
 }
 
 void RtpDataChannel::GetSrtpCryptoSuites_n(
     std::vector<int>* crypto_suites) const {
   GetSupportedDataCryptoSuites(crypto_options(), crypto_suites);
 }
 
 }  // namespace cricket