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

ui/app_list/app_list_model.cc

 // 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.
 
 #include "ui/app_list/app_list_model.h"
 
 namespace app_list {
 
 AppListModel::AppListModel() {
 }
 
 AppListModel::~AppListModel() {
 }
 
 void AppListModel::AddItem(AppListItemModel* item) {
   items_.Add(item);
 }
 
-void AppListModel::AddItemAt(int index, AppListItemModel* item) {
+void AppListModel::AddItemAt(size_t index, AppListItemModel* item) {
   items_.AddAt(index, item);
 }
 
-void AppListModel::DeleteItemAt(int index) {
+void AppListModel::DeleteItemAt(size_t index) {
   items_.DeleteAt(index);
 }
 
-AppListItemModel* AppListModel::GetItemAt(int index) {
+AppListItemModel* AppListModel::GetItemAt(size_t index) {
   return items_.GetItemAt(index);
 }
 
 void AppListModel::AddObserver(ui::ListModelObserver* observer) {
   items_.AddObserver(observer);
 }
 
 void AppListModel::RemoveObserver(ui::ListModelObserver* observer) {
   items_.RemoveObserver(observer);
 }
 
 }  // namespace app_list