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

chrome/browser/extensions/extension_browsertest.h

 // Copyright (c) 2012 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.
 
 #ifndef CHROME_BROWSER_EXTENSIONS_EXTENSION_BROWSERTEST_H_
 #define CHROME_BROWSER_EXTENSIONS_EXTENSION_BROWSERTEST_H_
 
 #include <string>
 
 #include "base/command_line.h"
@@ skipping 210 matching lines @@
                                     true);
   }
 
   // Begins install process but simulates a user cancel.
   const extensions::Extension* StartInstallButCancel(
       const base::FilePath& path) {
     return InstallOrUpdateExtension(
         std::string(), path, INSTALL_UI_TYPE_CANCEL, 0);
   }
 
-  void ReloadExtension(const std::string extension_id);
+  void ReloadExtension(const std::string& extension_id);
 
   void UnloadExtension(const std::string& extension_id);
 
   void UninstallExtension(const std::string& extension_id);
 
   void DisableExtension(const std::string& extension_id);
 
   void EnableExtension(const std::string& extension_id);
 
   // Wait for the number of visible page actions to change to |count|.
@@ skipping 159 matching lines @@
 #endif
 
   // The default profile to be used.
   Profile* profile_;
 
   // Cache cache implementation.
   scoped_ptr<extensions::ExtensionCacheFake> test_extension_cache_;
 };
 
 #endif  // CHROME_BROWSER_EXTENSIONS_EXTENSION_BROWSERTEST_H_