MD History: promote menu button to show clear browsing data in narrow mode

chrome/browser/ui/webui/md_history_ui.h

 // Copyright 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_MD_HISTORY_UI_H_
 #define CHROME_BROWSER_UI_WEBUI_MD_HISTORY_UI_H_
 
 #include "base/macros.h"
 #include "content/public/browser/web_ui_controller.h"
 #include "ui/base/layout.h"
 
 class Profile;
 
 namespace base {
+class ListValue;
 class RefCountedMemory;
 }
 
+namespace content {
+class WebUIDataSource;
+}
+
+namespace user_prefs {
+class PrefRegistrySyncable;
+}
+
 class MdHistoryUI : public content::WebUIController {
  public:
   explicit MdHistoryUI(content::WebUI* web_ui);
   ~MdHistoryUI() override;
 
   static bool IsEnabled(Profile* profile);
 
   // Reset the current list of features and explicitly set MD History enabled.
   static void SetEnabledForTesting(bool enabled);
 
+  static void RegisterProfilePrefs(user_prefs::PrefRegistrySyncable* registry);
+
   static base::RefCountedMemory* GetFaviconResourceBytes(
       ui::ScaleFactor scale_factor);
 
  private:
+  // Handler for the "menuPromoShown" message from the page. No arguments.
+  void HandleMenuPromoShown(const base::ListValue* args);
+
+  content::WebUIDataSource* data_source_;  // weak
+
   DISALLOW_COPY_AND_ASSIGN(MdHistoryUI);
 };
 
 #endif  // CHROME_BROWSER_UI_WEBUI_MD_HISTORY_UI_H_