message: string -> const string&
diff: @@ -2561,7 +2561,7 @@ TEST_F(OpConverterTest, ConvertActivation) {
  ops_to_test.push_back("Relu6");
  ops_to_test.push_back("LeakyRelu");
  // Ok.
  - for (string op_name : ops_to_test) {
  + for (const string& op_name : ops_to_test) {
  Reset();
  NodeDef node_def = get_act_nodedef(op_name);
  AddTestTensor("input", {1, 2, 3});
  @@ -4093,7 +4093,7 @@ TEST_F(OpConverterTest, ConvertUnary) {
  // Add other unary ops to test.
  ops_to_test.push_back("Rsqrt");
  // Ok.
  - for (string op_name : ops_to_test) {
  + for (const string& op_name : ops_to_test) {
  Reset();
  NodeDef node_def = get_unary_nodedef(op_name);
  AddTestTensor("input", {1, 2, 3});
  