Log how often DTLS negotiation failed because of incompatible ciphersuites.

webrtc/p2p/base/transportcontroller.h

Index: webrtc/p2p/base/transportcontroller.h
diff --git a/webrtc/p2p/base/transportcontroller.h b/webrtc/p2p/base/transportcontroller.h
index da3bab3fe44f7fe26ece30cfb6129f2a7c2a7f37..a32620281a9bd2b99bbe1a913e05e99b1f0665f1 100644
--- a/webrtc/p2p/base/transportcontroller.h
+++ b/webrtc/p2p/base/transportcontroller.h
@@ -119,6 +119,8 @@ class TransportController : public sigslot::has_slots<>,
   // for unit test
   const rtc::scoped_refptr<rtc::RTCCertificate>& certificate_for_testing();
 
+  sigslot::signal1<rtc::SSLHandshakeError> SignalDtlsHandshakeError;
+
  protected:
   // Protected and virtual so we can override it in unit tests.
   virtual Transport* CreateTransport_n(const std::string& transport_name);
@@ -202,6 +204,8 @@ class TransportController : public sigslot::has_slots<>,
 
   void UpdateAggregateStates_n();
 
+  void OnDtlsHandshakeError(rtc::SSLHandshakeError error);
+
   rtc::Thread* const signaling_thread_ = nullptr;
   rtc::Thread* const network_thread_ = nullptr;
   typedef std::map<std::string, Transport*> TransportMap;