Introduce ui.dll / libui.so for the component build.

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/ui_api.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.
-int MessageBox(HWND hwnd,
-               const string16& text,
-               const string16& caption,
-               UINT flags);
+UI_API int MessageBox(HWND hwnd,
+                      const string16& text,
+                      const string16& caption,
+                      UINT flags);
 
 }  // namespace ui
 
 #endif  // UI_BASE_MESSAGE_BOX_WIN_H_