Add --use-more-webui runtime flag to toggle WebUI replacements for native dialogs.

chrome/browser/ui/webui/chrome_web_ui.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 CHROME_BROWSER_UI_WEBUI_CHROME_WEB_UI_H_
 #define CHROME_BROWSER_UI_WEBUI_CHROME_WEB_UI_H_
 #pragma once
 
 #include "content/browser/webui/web_ui.h"
 
 class Profile;
 
 class ChromeWebUI : public WebUI {
  public:
   explicit ChromeWebUI(TabContents* contents);
   virtual ~ChromeWebUI();
 
   // Returns the profile for this WebUI.
   Profile* GetProfile() const;
 
   // Returns true if the bookmark bar should be forced to being visible,
   // overriding the user's preference.
   bool force_bookmark_bar_visible() const {
     return force_bookmark_bar_visible_;
   }
 
+  // IsMoreWebUI returns a command line flag that tracks whether to use
+  // available WebUI implementations of native dialogs.
+  static bool IsMoreWebUI();
+
  protected:
   void set_force_bookmark_bar_visible(bool value) {
     force_bookmark_bar_visible_ = value;
   }
 
  private:
   bool force_bookmark_bar_visible_;
 
   DISALLOW_COPY_AND_ASSIGN(ChromeWebUI);
 };
 
 #endif  // CHROME_BROWSER_UI_WEBUI_CHROME_WEB_UI_H_