Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

components/nacl/renderer/nacl_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 COMPONENTS_NACL_RENDERER_NACL_HELPER_H_
 #define COMPONENTS_NACL_RENDERER_NACL_HELPER_H_
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "content/public/renderer/render_frame_observer.h"
 
 namespace nacl {
 
 // This class listens for Pepper creation events from the RenderFrame. For the
 // NaCl trusted plugin, it configures it as an external plugin host.
 // TODO(dmichael): When the trusted plugin goes away, we need to figure out the
 //                 right event to watch for.
 class NaClHelper : public content::RenderFrameObserver {
  public:
   explicit NaClHelper(content::RenderFrame* render_frame);
   ~NaClHelper() override;
 
   // RenderFrameObserver.
   void DidCreatePepperPlugin(content::RendererPpapiHost* host) override;
 
  private:
+  // RenderFrameObserver implementation.
+  void OnDestruct() override;
+
   DISALLOW_COPY_AND_ASSIGN(NaClHelper);
 };
 
 }  // namespace nacl
 
 #endif  // COMPONENTS_NACL_RENDERER_NACL_HELPER_H_