Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

chromecast/renderer/cast_media_load_deferrer.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 CHROMECAST_RENDERER_CAST_MEDIA_LOAD_DEFERRER_H_
 #define CHROMECAST_RENDERER_CAST_MEDIA_LOAD_DEFERRER_H_
 
 #include "base/callback.h"
 #include "base/macros.h"
 #include "content/public/renderer/render_frame_observer.h"
 
 namespace chromecast {
 
 // Defers media player loading in background web apps until brought into
 // foreground.
 class CastMediaLoadDeferrer : public content::RenderFrameObserver {
  public:
   // Will run |closure| to continue loading the media resource once the page is
   // swapped in.
   CastMediaLoadDeferrer(content::RenderFrame* render_frame,
                         const base::Closure& continue_loading_cb);
   ~CastMediaLoadDeferrer() override;
 
  private:
   // content::RenderFrameObserver implementation:
   void WasShown() override;
+  void OnDestruct() override;
 
   base::Closure continue_loading_cb_;
 
   DISALLOW_COPY_AND_ASSIGN(CastMediaLoadDeferrer);
 };
 
 }  // namespace chromecast
 
 #endif  // CHROMECAST_RENDERER_CAST_MEDIA_LOAD_DEFERRER_H_