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

chrome/browser/ui/toolbar/toolbar_actions_model.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_UI_TOOLBAR_TOOLBAR_ACTIONS_MODEL_H_
 #define CHROME_BROWSER_UI_TOOLBAR_TOOLBAR_ACTIONS_MODEL_H_
 
 #include "base/compiler_specific.h"
 #include "base/memory/scoped_vector.h"
 #include "base/observer_list.h"
@@ skipping 38 matching lines @@
   // The different types of actions.
   enum ActionType {
     UNKNOWN_ACTION,
     COMPONENT_ACTION,
     EXTENSION_ACTION,
   };
 
   // An action id and its corresponding ActionType.
   struct ToolbarItem {
     ToolbarItem() : type(UNKNOWN_ACTION) {}
-    ToolbarItem(std::string action_id, ActionType action_type)
+    ToolbarItem(const std::string& action_id, ActionType action_type)
         : id(action_id), type(action_type) {}
 
     bool operator==(const ToolbarItem& other) { return other.id == id; }
 
     std::string id;
     ActionType type;
   };
 
   ToolbarActionsModel(Profile* profile,
                       extensions::ExtensionPrefs* extension_prefs);
@@ skipping 217 matching lines @@
   // For observing change of toolbar order preference by external entity (sync).
   PrefChangeRegistrar pref_change_registrar_;
   base::Closure pref_change_callback_;
 
   base::WeakPtrFactory<ToolbarActionsModel> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ToolbarActionsModel);
 };
 
 #endif  // CHROME_BROWSER_UI_TOOLBAR_TOOLBAR_ACTIONS_MODEL_H_