Log how often DTLS negotiation failed because of incompatible ciphersuites.

webrtc/p2p/base/transportcontroller.cc

Index: webrtc/p2p/base/transportcontroller.cc
diff --git a/webrtc/p2p/base/transportcontroller.cc b/webrtc/p2p/base/transportcontroller.cc
index b5021cceccb73c18592b7a8f8bd7d97861e09cce..1a13ca14b403c7dcb684bb201afbc43bffca3321 100644
--- a/webrtc/p2p/base/transportcontroller.cc
+++ b/webrtc/p2p/base/transportcontroller.cc
@@ -191,6 +191,8 @@ TransportChannel* TransportController::CreateTransportChannel_n(
       this, &TransportController::OnChannelRoleConflict_n);
   channel->SignalStateChanged.connect(
       this, &TransportController::OnChannelStateChanged_n);
+  channel->SignalDtlsHandshakeError.connect(
+      this, &TransportController::OnDtlsHandshakeError);
   channels_.insert(channels_.end(), RefCountedChannel(channel))->AddRef();
   // Adding a channel could cause aggregate state to change.
   UpdateAggregateStates_n();
@@ -685,4 +687,8 @@ void TransportController::UpdateAggregateStates_n() {
   }
 }
 
+void TransportController::OnDtlsHandshakeError(rtc::SSLHandshakeError error) {
+  SignalDtlsHandshakeError(error);
+}
+
 }  // namespace cricket