Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

components/dom_distiller/content/renderer/distiller_js_render_frame_observer.h

 // Copyright 2015 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_DOM_DISTILLER_CONTENT_RENDERER_DISTILLER_JS_RENDER_FRAME_OBSERVER_H_
 #define COMPONENTS_DOM_DISTILLER_CONTENT_RENDERER_DISTILLER_JS_RENDER_FRAME_OBSERVER_H_
 
 #include "base/memory/weak_ptr.h"
 #include "components/dom_distiller/content/common/distiller_javascript_service.mojom.h"
 #include "components/dom_distiller/content/renderer/distiller_native_javascript.h"
@@ skipping 24 matching lines @@
 
   // Add the mojo service to a RenderFrame's service registry.
   void RegisterMojoService();
   // Flag the current page as a distiller page.
   void SetIsDistillerPage();
 
  private:
   void CreateDistillerPageNotifierService(
       mojo::InterfaceRequest<mojom::DistillerPageNotifierService> request);
 
+  // RenderFrameObserver implementation.
+  void OnDestruct() override;
+
   // The isolated world that the distiller object should be written to.
   int distiller_isolated_world_id_;
 
   // Track if the current page is distilled. This is needed for testing.
   bool is_distiller_page_;
 
   // mojo::StrongBinding<mojom::DistillerPageNotifierService> binding_;
 
   // Handle to "distiller" JavaScript object functionality.
   std::unique_ptr<DistillerNativeJavaScript> native_javascript_handle_;
   base::WeakPtrFactory<DistillerJsRenderFrameObserver> weak_factory_;
 };
 
 }  // namespace dom_distiller
 
 #endif  // COMPONENTS_DOM_DISTILLER_CONTENT_RENDERER_DISTILLER_JS_RENDER_FRAME_OBSERVER_H_