Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

extensions/renderer/extension_helper.h

 // Copyright 2014 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 EXTENSIONS_RENDERER_EXTENSION_HELPER_H_
 #define EXTENSIONS_RENDERER_EXTENSION_HELPER_H_
 
 #include <string>
 
 #include "base/macros.h"
 #include "content/public/renderer/render_view_observer.h"
 
 namespace extensions {
 class Dispatcher;
 
 // RenderView-level plumbing for extension features.
 class ExtensionHelper : public content::RenderViewObserver {
  public:
   ExtensionHelper(content::RenderView* render_view, Dispatcher* dispatcher);
   ~ExtensionHelper() override;
 
  private:
   // RenderViewObserver implementation.
   bool OnMessageReceived(const IPC::Message& message) override;
   void DraggableRegionsChanged(blink::WebFrame* frame) override;
+  void OnDestruct() override;
 
   void OnAppWindowClosed();
   void OnSetFrameName(const std::string& name);
 
   Dispatcher* dispatcher_;
 
   DISALLOW_COPY_AND_ASSIGN(ExtensionHelper);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_RENDERER_EXTENSION_HELPER_H_