Add a new GetTlsUniqueChannelBinding method to SSLSocket, and implement nss version.

net/socket/ssl_client_socket_mac.cc

Index: net/socket/ssl_client_socket_mac.cc
diff --git a/net/socket/ssl_client_socket_mac.cc b/net/socket/ssl_client_socket_mac.cc
index ff43849b917add382fa2cde0b236f3c383cca03f..fb2e97a7016cbdae9692cb127fe6f4ec16905b39 100644
--- a/net/socket/ssl_client_socket_mac.cc
+++ b/net/socket/ssl_client_socket_mac.cc
@@ -789,6 +789,10 @@ int SSLClientSocketMac::ExportKeyingMaterial(const base::StringPiece& label,
   return ERR_NOT_IMPLEMENTED;
 }
 
+int SSLClientSocketMac::GetTLSUniqueChannelBinding(std::string* out) {
+  return ERR_NOT_IMPLEMENTED;
+}
+
 SSLClientSocket::NextProtoStatus
 SSLClientSocketMac::GetNextProto(std::string* proto,
                                  std::string* server_protos) {