Re-land MD Settings fix for crash on back/forward navigation

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

 // Copyright (c) 2015 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_MD_SETTINGS_UI_H_
 #define CHROME_BROWSER_UI_WEBUI_SETTINGS_MD_SETTINGS_UI_H_
 
 #include <unordered_set>
 
 #include "base/macros.h"
 #include "base/time/time.h"
 #include "content/public/browser/web_contents_observer.h"
 #include "content/public/browser/web_ui_controller.h"
 
+class GURL;
+
 namespace settings {
 
 class SettingsPageUIHandler;
 
 // The WebUI handler for chrome://md-settings.
 class MdSettingsUI : public content::WebUIController,
                      public content::WebContentsObserver {
  public:
-  explicit MdSettingsUI(content::WebUI* web_ui);
+  MdSettingsUI(content::WebUI* web_ui, const GURL& url);
   ~MdSettingsUI() override;
 
   // content::WebContentsObserver:
   void DidStartProvisionalLoadForFrame(
       content::RenderFrameHost* render_frame_host,
       const GURL& validated_url,
       bool is_error_page,
       bool is_iframe_srcdoc) override;
   void DocumentLoadedInFrame(
       content::RenderFrameHost *render_frame_host) override;
   void DocumentOnLoadCompletedInMainFrame() override;
 
  private:
   void AddSettingsPageUIHandler(SettingsPageUIHandler* handler);
 
   // Weak references; all |handlers_| are owned by |web_ui()|.
   std::unordered_set<SettingsPageUIHandler*> handlers_;
 
   base::Time load_start_time_;
 
   DISALLOW_COPY_AND_ASSIGN(MdSettingsUI);
 };
 
 }  // namespace settings
 
 #endif  // CHROME_BROWSER_UI_WEBUI_SETTINGS_MD_SETTINGS_UI_H_