Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

extensions/renderer/extensions_render_frame_observer.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 EXTENSIONS_RENDERER_EXTENSIONS_RENDER_FRAME_OBSERVER_H_
 #define EXTENSIONS_RENDERER_EXTENSIONS_RENDER_FRAME_OBSERVER_H_
 
 #include <stdint.h>
 
 #include "base/macros.h"
@@ skipping 10 matching lines @@
       content::RenderFrame* render_frame);
   ~ExtensionsRenderFrameObserver() override;
 
  private:
   // RenderFrameObserver implementation.
   void DetailedConsoleMessageAdded(const base::string16& message,
                                    const base::string16& source,
                                    const base::string16& stack_trace,
                                    uint32_t line_number,
                                    int32_t severity_level) override;
+  void OnDestruct() override;
 
   DISALLOW_COPY_AND_ASSIGN(ExtensionsRenderFrameObserver);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_RENDERER_EXTENSIONS_RENDER_FRAME_OBSERVER_H_