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

chrome/browser/sync_file_system/logger_unittest.cc

Index: chrome/browser/sync_file_system/logger_unittest.cc
diff --git a/chrome/browser/sync_file_system/logger_unittest.cc b/chrome/browser/sync_file_system/logger_unittest.cc
index aad548ab221281079c2bdaa884a34f1f1061d94b..78df07101756080a3109e8f39edf227c62dd3249 100644
--- a/chrome/browser/sync_file_system/logger_unittest.cc
+++ b/chrome/browser/sync_file_system/logger_unittest.cc
@@ -18,7 +18,8 @@ void LogSampleEvents() {
   util::Log(logging::LOG_ERROR, FROM_HERE, "Error test message");
 }
 
-bool ContainsString(std::string contains_string, EventLogger::Event event) {
+bool ContainsString(const std::string& contains_string,
+                    EventLogger::Event event) {
   return event.what.find(contains_string) != std::string::npos;
 }