Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

chrome/renderer/pepper/pepper_helper.h

 // Copyright (c) 2012 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_PEPPER_PEPPER_HELPER_H_
 #define CHROME_RENDERER_PEPPER_PEPPER_HELPER_H_
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "content/public/renderer/render_frame_observer.h"
 
 // This class listens for Pepper creation events from the RenderFrame and
 // attaches the parts required for Chrome-specific plugin support.
 class PepperHelper : public content::RenderFrameObserver {
  public:
   explicit PepperHelper(content::RenderFrame* render_frame);
   ~PepperHelper() override;
 
   // RenderFrameObserver.
   void DidCreatePepperPlugin(content::RendererPpapiHost* host) override;
+  void OnDestruct() override;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(PepperHelper);
 };
 
 #endif  // CHROME_RENDERER_PEPPER_PEPPER_HELPER_H_