Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

content/shell/renderer/shell_render_view_observer.h

 // Copyright 2013 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 CONTENT_SHELL_RENDERER_SHELL_RENDER_VIEW_OBSERVER_H_
 #define CONTENT_SHELL_RENDERER_SHELL_RENDER_VIEW_OBSERVER_H_
 
 #include "base/macros.h"
 #include "content/public/renderer/render_view_observer.h"
 
 namespace blink {
 class WebLocalFrame;
 }
 
 namespace content {
 
 class RenderView;
 
 
 class ShellRenderViewObserver : public RenderViewObserver {
  public:
   explicit ShellRenderViewObserver(RenderView* render_view);
   ~ShellRenderViewObserver() override;
 
  private:
   // RenderViewObserver implementation.
   void DidClearWindowObject(blink::WebLocalFrame* frame) override;
+  void OnDestruct() override;
 
   DISALLOW_COPY_AND_ASSIGN(ShellRenderViewObserver);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_SHELL_RENDERER_SHELL_RENDER_VIEW_OBSERVER_H_