Log how often DTLS negotiation failed because of incompatible ciphersuites.

webrtc/p2p/base/transportchannelimpl.h

Index: webrtc/p2p/base/transportchannelimpl.h
diff --git a/webrtc/p2p/base/transportchannelimpl.h b/webrtc/p2p/base/transportchannelimpl.h
index 59065a10a8258d93e1416588ff38f187036e2441..910ec3133e47b99215fd61f79d15b8a4e127840f 100644
--- a/webrtc/p2p/base/transportchannelimpl.h
+++ b/webrtc/p2p/base/transportchannelimpl.h
@@ -100,6 +100,9 @@ class TransportChannelImpl : public TransportChannel {
   // Emitted whenever the transport channel state changed.
   sigslot::signal1<TransportChannelImpl*> SignalStateChanged;
 
+  // Emitted whenever the Dtls handshake failed on some transport channel.
+  sigslot::signal1<rtc::SSLHandshakeError> SignalDtlsHandshakeError;
+
  private:
   RTC_DISALLOW_COPY_AND_ASSIGN(TransportChannelImpl);
 };