MD Settings: Convert C++ handlers to be JavaScript-lifecycle aware.

chrome/browser/ui/webui/settings/settings_page_ui_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_SETTINGS_PAGE_UI_HANDLER_H_
 #define CHROME_BROWSER_UI_WEBUI_SETTINGS_SETTINGS_PAGE_UI_HANDLER_H_
 
 #include "base/macros.h"
 #include "base/values.h"
 #include "content/public/browser/web_ui_message_handler.h"
@@ skipping 13 matching lines @@
   void ResolveJavascriptCallback(const base::Value& callback_id,
                                  const base::Value& response);
 
   // Helper method for responding to JS requests initiated with
   // cr.sendWithPromise(), for the case where the returned promise should be
   // rejected (request failed).
   void RejectJavascriptCallback(const base::Value& callback_id,
                                 const base::Value& response);
 
  private:
+  // SettingsPageUIHandler subclasses must be JavaScript-lifecycle safe.
+  void OnJavascriptAllowed() override = 0;
+  void OnJavascriptDisallowed() override = 0;
+
   DISALLOW_COPY_AND_ASSIGN(SettingsPageUIHandler);
 };
 
 }  // namespace settings
 
 #endif  // CHROME_BROWSER_UI_WEBUI_SETTINGS_SETTINGS_PAGE_UI_HANDLER_H_