Cleanup: Pass std::string as const reference if possible

content/common/cc_messages_perftest.cc

Index: content/common/cc_messages_perftest.cc
diff --git a/content/common/cc_messages_perftest.cc b/content/common/cc_messages_perftest.cc
index 78085d21f5606a590e7f24970f153cd878f05c04..0251354ce98c10243a45cba3795343b04b85934a 100644
--- a/content/common/cc_messages_perftest.cc
+++ b/content/common/cc_messages_perftest.cc
@@ -27,7 +27,8 @@ static const int kTimeCheckInterval = 10;
 
 class CCMessagesPerfTest : public testing::Test {
  protected:
-  static void RunTest(std::string test_name, const CompositorFrame& frame) {
+  static void RunTest(const std::string& test_name,
+                      const CompositorFrame& frame) {
     for (int i = 0; i < kNumWarmupRuns; ++i) {
       IPC::Message msg(1, 2, IPC::Message::PRIORITY_NORMAL);
       IPC::ParamTraits<CompositorFrame>::Write(&msg, frame);