ui: Change ListModel to operate in size_t instead of int.

ui/app_list/app_list_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 UI_APP_LIST_APP_LIST_MODEL_H_
 #define UI_APP_LIST_APP_LIST_MODEL_H_
 #pragma once
 
 #include "base/basictypes.h"
 #include "ui/app_list/app_list_export.h"
 #include "ui/app_list/app_list_item_model.h"
 #include "ui/base/models/list_model.h"
 
 namespace app_list {
 
 // Model for AppListModelView that owns a list of AppListItemModel.
 class APP_LIST_EXPORT AppListModel {
  public:
   AppListModel();
   ~AppListModel();
 
   // Adds an item to the model. The model takes ownership of |item|.
   void AddItem(AppListItemModel* item);
-  void AddItemAt(int index, AppListItemModel* item);
+  void AddItemAt(size_t index, AppListItemModel* item);
 
-  void DeleteItemAt(int index);
+  void DeleteItemAt(size_t index);
 
-  AppListItemModel* GetItemAt(int index);
+  AppListItemModel* GetItemAt(size_t index);
 
   void AddObserver(ui::ListModelObserver* observer);
   void RemoveObserver(ui::ListModelObserver* observer);
 
-  int item_count() const { return items_.item_count(); }
+  size_t item_count() const { return items_.item_count(); }
 
  private:
   typedef ui::ListModel<AppListItemModel> Items;
   Items items_;
 
   DISALLOW_COPY_AND_ASSIGN(AppListModel);
 };
 
 }  // namespace app_list
 
 #endif  // UI_APP_LIST_APP_LIST_MODEL_H_