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

webrtc/pc/channel.cc

Index: webrtc/pc/channel.cc
diff --git a/webrtc/pc/channel.cc b/webrtc/pc/channel.cc
index f242f17bd23b0efdef5ac76a7ba7f1dce6dbab82..df26d4cdb818483980fffff344e85015d716f454 100644
--- a/webrtc/pc/channel.cc
+++ b/webrtc/pc/channel.cc
@@ -1183,6 +1183,15 @@ 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: