Log how often DTLS negotiation failed because of incompatible ciphersuites.

webrtc/api/webrtcsession.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 469 matching lines @@
   transport_controller_->SignalConnectionState.connect(
       this, &WebRtcSession::OnTransportControllerConnectionState);
   transport_controller_->SignalReceiving.connect(
       this, &WebRtcSession::OnTransportControllerReceiving);
   transport_controller_->SignalGatheringState.connect(
       this, &WebRtcSession::OnTransportControllerGatheringState);
   transport_controller_->SignalCandidatesGathered.connect(
       this, &WebRtcSession::OnTransportControllerCandidatesGathered);
   transport_controller_->SignalCandidatesRemoved.connect(
       this, &WebRtcSession::OnTransportControllerCandidatesRemoved);
+  transport_controller_->SignalDtlsHandshakeError.connect(
+      this, &WebRtcSession::OnDtlsHandshakeError);
 }
 
 WebRtcSession::~WebRtcSession() {
   ASSERT(signaling_thread()->IsCurrent());
   // Destroy video_channel_ first since it may have a pointer to the
   // voice_channel_.
   if (video_channel_) {
     SignalVideoChannelDestroyed();
     channel_manager_->DestroyVideoChannel(video_channel_.release());
   }
@@ skipping 1501 matching lines @@
     metrics_observer_->IncrementSparseEnumCounter(ssl_counter_type,
                                                   ssl_cipher_suite);
   }
 }
 
 void WebRtcSession::OnSentPacket_w(const rtc::SentPacket& sent_packet) {
   RTC_DCHECK(worker_thread()->IsCurrent());
   media_controller_->call_w()->OnSentPacket(sent_packet);
 }
 
+void WebRtcSession::OnDtlsHandshakeError(rtc::SSLHandshakeError error) {
+  metrics_observer_->IncrementEnumCounter(

[skvlad, 2016/07/22 01:25:14]

Please add a DCHECK(metrics_observer_ != NULL)

[honghaiz3, 2016/07/22 21:35:26]

I think you should add a condition on it. 
if (metrics_observer_) {
...

+      webrtc::kEnumCounterDtlsHandshakeError, static_cast<int>(error),
+      static_cast<int>(rtc::SSLHandshakeError::ERR_OTHER) + 1);
+}
+
 }  // namespace webrtc