Log how often DTLS negotiation failed because of incompatible ciphersuites.

webrtc/api/webrtcsession.cc

Index: webrtc/api/webrtcsession.cc
diff --git a/webrtc/api/webrtcsession.cc b/webrtc/api/webrtcsession.cc
index 8c83dbbc62a414cd304c44409c9630d4e08887f4..3e0f33d609142292d128bb95b6658e506bc11b81 100644
--- a/webrtc/api/webrtcsession.cc
+++ b/webrtc/api/webrtcsession.cc
@@ -487,6 +487,8 @@ WebRtcSession::WebRtcSession(webrtc::MediaControllerInterface* media_controller,
       this, &WebRtcSession::OnTransportControllerCandidatesGathered);
   transport_controller_->SignalCandidatesRemoved.connect(
       this, &WebRtcSession::OnTransportControllerCandidatesRemoved);
+  transport_controller_->SignalDtlsHandshakeError.connect(
+      this, &WebRtcSession::OnDtlsHandshakeError);
 }
 
 WebRtcSession::~WebRtcSession() {
@@ -2008,4 +2010,10 @@ void WebRtcSession::OnSentPacket_w(const rtc::SentPacket& sent_packet) {
   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