app_list: Another pass of cpplint.py to make it clean.

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_
 
+#include <string>
+
 #include "base/basictypes.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/observer_list.h"
 #include "ui/app_list/app_list_export.h"
 #include "ui/app_list/app_list_item_list.h"
 #include "ui/app_list/app_list_item_list_observer.h"
 #include "ui/base/models/list_model.h"
 
 namespace app_list {
 
@@ skipping 137 matching lines @@
 
   Status status_;
   ObserverList<AppListModelObserver> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(AppListModel);
 };
 
 }  // namespace app_list
 
 #endif  // UI_APP_LIST_APP_LIST_MODEL_H_