Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

content/public/renderer/render_view_observer.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 CONTENT_PUBLIC_RENDERER_RENDER_VIEW_OBSERVER_H_
 #define CONTENT_PUBLIC_RENDERER_RENDER_VIEW_OBSERVER_H_
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "content/common/content_export.h"
@@ skipping 13 matching lines @@
 namespace content {
 
 class RenderView;
 class RenderViewImpl;
 
 // Base class for objects that want to filter incoming IPCs, and also get
 // notified of changes to the frame.
 class CONTENT_EXPORT RenderViewObserver : public IPC::Listener,
                                           public IPC::Sender {
  public:
-  // By default, observers will be deleted when the RenderView goes away.  If
-  // they want to outlive it, they can override this function.
-  virtual void OnDestruct();
+  // A subclass can use this to delete itself. If it does not, the subclass must
+  // always null-check each call to render_view() becase the RenderView can
+  // go away at any time.
+  virtual void OnDestruct() = 0;
 
   // These match the WebKit API notifications
   virtual void DidStartLoading() {}
   virtual void DidStopLoading() {}
   virtual void DidFinishDocumentLoad(blink::WebLocalFrame* frame) {}
   virtual void DidFailLoad(blink::WebLocalFrame* frame,
                            const blink::WebURLError& error) {}
   virtual void DidFinishLoad(blink::WebLocalFrame* frame) {}
   virtual void DidStartProvisionalLoad(blink::WebLocalFrame* frame) {}
   virtual void DidFailProvisionalLoad(blink::WebLocalFrame* frame,
@@ skipping 59 matching lines @@
   RenderViewImpl* render_view_;
   // The routing ID of the associated RenderView.
   int routing_id_;
 
   DISALLOW_COPY_AND_ASSIGN(RenderViewObserver);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_RENDERER_RENDER_VIEW_OBSERVER_H_