Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

chrome/renderer/prerender/prerender_helper.h

 // Copyright (c) 2011 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_RENDERER_PRERENDER_PRERENDER_HELPER_H_
 #define CHROME_RENDERER_PRERENDER_PRERENDER_HELPER_H_
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "base/time/time.h"
 #include "content/public/renderer/render_frame_observer.h"
 #include "content/public/renderer/render_frame_observer_tracker.h"
 
 namespace prerender {
 
 // Helper class to track whether its RenderFrame is currently being prerendered.
 // Created when prerendering starts and deleted as soon as it stops.
 class PrerenderHelper
     : public content::RenderFrameObserver,
       public content::RenderFrameObserverTracker<PrerenderHelper> {
  public:
   explicit PrerenderHelper(content::RenderFrame* render_frame);
   ~PrerenderHelper() override;
 
   // Returns true if |render_view| is currently prerendering.
   static bool IsPrerendering(const content::RenderFrame* render_frame);
 
  private:
-  // RenderViewObserver implementation
+  // RenderFrameObserver implementation.
   bool OnMessageReceived(const IPC::Message& message) override;
+  void OnDestruct() override;
 
   void OnSetIsPrerendering(bool is_prerendering);
 
   DISALLOW_COPY_AND_ASSIGN(PrerenderHelper);
 };
 
 }  // namespace prerender
 
 #endif  // CHROME_RENDERER_PRERENDER_PRERENDER_HELPER_H_