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

testing/multiprocess_func_list.cc

Index: testing/multiprocess_func_list.cc
diff --git a/testing/multiprocess_func_list.cc b/testing/multiprocess_func_list.cc
index 49ae07dd3e8d99d57ba806851b9e53474c95128c..f96c2b5079a254f30e220f68077139f7feea3ab7 100644
--- a/testing/multiprocess_func_list.cc
+++ b/testing/multiprocess_func_list.cc
@@ -40,7 +40,7 @@ AppendMultiProcessTest::AppendMultiProcessTest(
       ProcessFunctions(main_func_ptr, setup_func_ptr);
 }
 
-int InvokeChildProcessTest(std::string test_name) {
+int InvokeChildProcessTest(const std::string& test_name) {
   MultiProcessTestMap& func_lookup_table = GetMultiprocessFuncMap();
   MultiProcessTestMap::iterator it = func_lookup_table.find(test_name);
   if (it != func_lookup_table.end()) {