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

Passing std::string by reference can prevent extra copying of object.

BUG=367418
TEST=
[email protected]

Review-Url: https://codereview.chromium.org/2001093008
Cr-Commit-Position: refs/heads/master@{#396030}
diff --git a/testing/multiprocess_func_list.cc b/testing/multiprocess_func_list.cc
index 49ae07d..f96c2b5 100644
--- a/testing/multiprocess_func_list.cc
+++ b/testing/multiprocess_func_list.cc
@@ -40,7 +40,7 @@
       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()) {