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

chrome/browser/extensions/extension_message_bubble_controller.h

 // Copyright (c) 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.
 
 #ifndef CHROME_BROWSER_EXTENSIONS_EXTENSION_MESSAGE_BUBBLE_CONTROLLER_H_
 #define CHROME_BROWSER_EXTENSIONS_EXTENSION_MESSAGE_BUBBLE_CONTROLLER_H_
 
 #include <string>
 #include "chrome/browser/extensions/extension_message_bubble.h"
 #include "extensions/browser/browser_context_keyed_api_factory.h"
@@ skipping 66 matching lines @@
     virtual bool HasBubbleInfoBeenAcknowledged(const std::string& extension_id);
     virtual void SetBubbleInfoBeenAcknowledged(const std::string& extension_id,
                                                bool value);
 
    protected:
     Profile* profile() { return profile_; }
     ExtensionService* service() { return service_; }
     const ExtensionRegistry* registry() const { return registry_; }
 
     std::string get_acknowledged_flag_pref_name() const;
-    void set_acknowledged_flag_pref_name(std::string pref_name);
+    void set_acknowledged_flag_pref_name(const std::string& pref_name);
 
    private:
     // A weak pointer to the profile we are associated with. Not owned by us.
     Profile* profile_;
 
     // The extension service associated with the profile.
     ExtensionService* service_;
 
     // The extension registry associated with the profile.
     ExtensionRegistry* registry_;
@@ skipping 67 matching lines @@
 
   // Whether or not the bubble is highlighting extensions.
   bool did_highlight_;
 
   DISALLOW_COPY_AND_ASSIGN(ExtensionMessageBubbleController);
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_EXTENSION_MESSAGE_BUBBLE_CONTROLLER_H_