Log how often DTLS negotiation failed because of incompatible ciphersuites.

webrtc/p2p/base/dtlstransportchannel.cc

Index: webrtc/p2p/base/dtlstransportchannel.cc
diff --git a/webrtc/p2p/base/dtlstransportchannel.cc b/webrtc/p2p/base/dtlstransportchannel.cc
index 6e8213c6066a9f8d493b688aaf48dad2755739fb..3dde8fea0535199e31c6ff75c4c9575d21b37b16 100644
--- a/webrtc/p2p/base/dtlstransportchannel.cc
+++ b/webrtc/p2p/base/dtlstransportchannel.cc
@@ -280,6 +280,8 @@ bool DtlsTransportChannelWrapper::SetupDtls() {
   dtls_->SetMaxProtocolVersion(ssl_max_version_);
   dtls_->SetServerRole(ssl_role_);
   dtls_->SignalEvent.connect(this, &DtlsTransportChannelWrapper::OnDtlsEvent);
+  dtls_->SignalSSLHandshakeError.connect(
+      this, &DtlsTransportChannelWrapper::OnDtlsHandshakeError);
   if (!dtls_->SetPeerCertificateDigest(
           remote_fingerprint_algorithm_,
           reinterpret_cast<unsigned char*>(remote_fingerprint_value_.data()),
@@ -676,4 +678,9 @@ void DtlsTransportChannelWrapper::OnChannelStateChanged(
   SignalStateChanged(this);
 }
 
+void DtlsTransportChannelWrapper::OnDtlsHandshakeError(
+    rtc::SSLHandshakeError error) {
+  SignalDtlsHandshakeError(error);
+}
+
 }  // namespace cricket