MD Settings: break SettingsPageUIHandler to its own file

chrome/browser/ui/webui/settings/system_handler.h

 // Copyright 2016 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 CHROME_BROWSER_UI_WEBUI_SETTINGS_SYSTEM_HANDLER_H_
 #define CHROME_BROWSER_UI_WEBUI_SETTINGS_SYSTEM_HANDLER_H_
 
 #include "base/macros.h"
-#include "chrome/browser/ui/webui/settings/md_settings_ui.h"
+#include "chrome/browser/ui/webui/settings/settings_page_ui_handler.h"
 
 namespace base {
 class ListValue;
 }
 
 namespace settings {
 
 class SystemHandler : public SettingsPageUIHandler {
  public:
   SystemHandler();
   ~SystemHandler() override;
 
   // SettingsPageUIHandler:
   void RegisterMessages() override;
 
  private:
   // Handler for the "changeProxySettings" message. No args.
   void HandleChangeProxySettings(const base::ListValue* /*args*/);
 
   DISALLOW_COPY_AND_ASSIGN(SystemHandler);
 };
 
 }  // namespace settings
 
 #endif  // CHROME_BROWSER_UI_WEBUI_SETTINGS_SYSTEM_HANDLER_H_