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

chrome/browser/sync_file_system/logger_unittest.cc

 // Copyright 2013 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.
 
 #include "chrome/browser/sync_file_system/logger.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 using drive::EventLogger;
 
 namespace sync_file_system {
 
 namespace {
 
 // Logs one event at each supported LogSeverity level.
 void LogSampleEvents() {
   util::Log(logging::LOG_INFO, FROM_HERE, "Info test message");
   util::Log(logging::LOG_WARNING, FROM_HERE, "Warning test message");
   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;
 }
 
 }  // namespace
 
 class LoggerTest : public testing::Test {
  public:
   LoggerTest() {}
 
   void SetUp() override {
@@ skipping 18 matching lines @@
 TEST_F(LoggerTest, ClearLog) {
   LogSampleEvents();
   EXPECT_EQ(3u, util::GetLogHistory().size());
 
   util::ClearLog();
   EXPECT_EQ(0u, util::GetLogHistory().size());
 }
 
 
 }  // namespace sync_file_system