Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

content/public/renderer/render_frame_observer.h

 // Copyright 2013 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_FRAME_OBSERVER_H_
 #define CONTENT_PUBLIC_RENDERER_RENDER_FRAME_OBSERVER_H_
 
 #include <stdint.h>
 
 #include "base/compiler_specific.h"
@@ skipping 19 matching lines @@
 
 class RendererPpapiHost;
 class RenderFrame;
 class RenderFrameImpl;
 
 // Base class for objects that want to filter incoming IPCs, and also get
 // notified of changes to the frame.
 class CONTENT_EXPORT RenderFrameObserver : public IPC::Listener,
                                            public IPC::Sender {
  public:
-  // By default, observers will be deleted when the RenderFrame 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_frame() becase the RenderFrame can
+  // go away at any time.
+  virtual void OnDestruct() = 0;
 
   // Called when a Pepper plugin is created.
   virtual void DidCreatePepperPlugin(RendererPpapiHost* host) {}
 
   // Called when a load is explicitly stopped by the user or browser.
   virtual void OnStop() {}
 
   // Called when the RenderFrame visiblity is changed.
   virtual void WasHidden() {}
   virtual void WasShown() {}
@@ skipping 91 matching lines @@
   RenderFrame* render_frame_;
   // The routing ID of the associated RenderFrame.
   int routing_id_;
 
   DISALLOW_COPY_AND_ASSIGN(RenderFrameObserver);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_RENDERER_RENDER_FRAME_OBSERVER_H_