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

net/socket/ssl_client_socket_openssl.cc

Index: net/socket/ssl_client_socket_openssl.cc
diff --git a/net/socket/ssl_client_socket_openssl.cc b/net/socket/ssl_client_socket_openssl.cc
index 85d0d650f42df1b99028b01f0f9597599989b252..8bdbfd11843721476e3b9fcb7fea71d428ffcb6b 100644
--- a/net/socket/ssl_client_socket_openssl.cc
+++ b/net/socket/ssl_client_socket_openssl.cc
@@ -663,6 +663,10 @@ int SSLClientSocketOpenSSL::ExportKeyingMaterial(
   return OK;
 }
 
+int SSLClientSocketOpenSSL::GetTLSUniqueChannelBinding(std::string* out) {
+  return ERR_NOT_IMPLEMENTED;
+}
+
 SSLClientSocket::NextProtoStatus SSLClientSocketOpenSSL::GetNextProto(
     std::string* proto, std::string* server_protos) {
   *proto = npn_proto_;