Cleanup: Pass std::string as const reference from testing/

testing/multiprocess_func_list.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef TESTING_MULTIPROCESS_FUNC_LIST_H_
 #define TESTING_MULTIPROCESS_FUNC_LIST_H_
 
 #include <string>
 
 // This file provides the plumbing to register functions to be executed
@@ skipping 29 matching lines @@
  public:
   // |main_func_ptr| is the main function that is run in the child process.
   // |setup_func_ptr| is a function run when the global mapping is added.
   AppendMultiProcessTest(std::string test_name,
                          TestMainFunctionPtr main_func_ptr,
                          SetupFunctionPtr setup_func_ptr);
 };
 
 // Invoke the main function of a test previously registered with
 // MULTIPROCESS_TEST_MAIN()
-int InvokeChildProcessTest(std::string test_name);
+int InvokeChildProcessTest(const std::string& test_name);
 
 // This macro creates a global MultiProcessTest::AppendMultiProcessTest object
 // whose constructor does the work of adding the global mapping.
 #define MULTIPROCESS_TEST_MAIN(test_main) \
   MULTIPROCESS_TEST_MAIN_WITH_SETUP(test_main, NULL)
 
 // Same as above but lets callers specify a setup method that is run in the
 // child process, just before the main function is run.  This facilitates
 // adding a generic one-time setup function for multiple tests.
 #define MULTIPROCESS_TEST_MAIN_WITH_SETUP(test_main, test_setup) \
   int test_main(); \
   namespace { \
     multi_process_function_list::AppendMultiProcessTest \
     AddMultiProcessTest##_##test_main(#test_main, (test_main), (test_setup)); \
   } \
   int test_main()
 
 }  // namespace multi_process_function_list
 
 #endif  // TESTING_MULTIPROCESS_FUNC_LIST_H_