Gather metrics classifying the cause of the TLS fallback.

net/socket/ssl_client_socket_nss.cc

Index: net/socket/ssl_client_socket_nss.cc
diff --git a/net/socket/ssl_client_socket_nss.cc b/net/socket/ssl_client_socket_nss.cc
index c9082bee6831205b3506e29e89b2d0cfe2e66073..9d3fd766ebeefcfdb7eb33b3527909136af95362 100644
--- a/net/socket/ssl_client_socket_nss.cc
+++ b/net/socket/ssl_client_socket_nss.cc
@@ -106,6 +106,7 @@
 #include "net/ssl/ssl_cert_request_info.h"
 #include "net/ssl/ssl_cipher_suite_names.h"
 #include "net/ssl/ssl_connection_status_flags.h"
+#include "net/ssl/ssl_failure_state.h"
 #include "net/ssl/ssl_info.h"
 
 #if defined(USE_NSS_CERTS)
@@ -3212,4 +3213,10 @@ ChannelIDService* SSLClientSocketNSS::GetChannelIDService() const {
   return channel_id_service_;
 }
 
+SSLFailureState SSLClientSocketNSS::GetSSLFailureState() const {
+  if (completed_handshake_)
+    return kSSLFailureNone;
+  return kSSLFailureUnknown;
+}
+
 }  // namespace net