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

chrome/browser/logging_chrome_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 "base/environment.h"
 #include "base/files/file_path.h"
 #include "base/memory/scoped_ptr.h"
 #include "chrome/common/env_vars.h"
 #include "chrome/common/logging_chrome.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 class ChromeLoggingTest : public testing::Test {
  public:
   // Stores the current value of the log file name environment
   // variable and sets the variable to new_value.
-  void SaveEnvironmentVariable(std::string new_value) {
+  void SaveEnvironmentVariable(const std::string& new_value) {
     scoped_ptr<base::Environment> env(base::Environment::Create());
     if (!env->GetVar(env_vars::kLogFileName, &environment_filename_))
       environment_filename_ = "";
 
     env->SetVar(env_vars::kLogFileName, new_value);
   }
 
   // Restores the value of the log file nave environment variable
   // previously saved by SaveEnvironmentVariable().
   void RestoreEnvironmentVariable() {
@@ skipping 19 matching lines @@
 // Tests the log file name getter with an environment variable.
 TEST_F(ChromeLoggingTest, EnvironmentLogFileName) {
   SaveEnvironmentVariable("test value");
 
   base::FilePath filename = logging::GetLogFileName();
   ASSERT_EQ(base::FilePath(FILE_PATH_LITERAL("test value")).value(),
             filename.value());
 
   RestoreEnvironmentVariable();
 }