ui/base: Move message_box_win.{h,cc} to ui/base/win.

ui/base/message_box_win.h

-// Copyright (c) 2011 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_BASE_MESSAGE_BOX_WIN_H_
-#define UI_BASE_MESSAGE_BOX_WIN_H_
-#pragma once
-
-#include <windows.h>
-
-#include "base/string16.h"
-#include "ui/base/ui_export.h"
-
-namespace ui {
-
-// A wrapper around Windows' MessageBox function. Using a Chrome specific
-// MessageBox function allows us to control certain RTL locale flags so that
-// callers don't have to worry about adding these flags when running in a
-// right-to-left locale.
-UI_EXPORT int MessageBox(HWND hwnd,
-                         const string16& text,
-                         const string16& caption,
-                         UINT flags);
-
-}  // namespace ui
-
-#endif  // UI_BASE_MESSAGE_BOX_WIN_H_