Assert that frame() derefs a non-null pointer.

Source/core/frame/FrameView.h

 /*
    Copyright (C) 1997 Martin Jones (mjones@kde.org)
              (C) 1998 Waldo Bastian (bastian@kde.org)
              (C) 1998, 1999 Torben Weis (weis@kde.org)
              (C) 1999 Lars Knoll (knoll@kde.org)
              (C) 1999 Antti Koivisto (koivisto@kde.org)
    Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
 
    This library is free software; you can redistribute it and/or
    modify it under the terms of the GNU Library General Public
@@ skipping 54 matching lines @@
     static PassRefPtr<FrameView> create(LocalFrame*);
     static PassRefPtr<FrameView> create(LocalFrame*, const IntSize& initialSize);
 
     virtual ~FrameView();
 
     virtual HostWindow* hostWindow() const OVERRIDE;
 
     virtual void invalidateRect(const IntRect&) OVERRIDE;
     virtual void setFrameRect(const IntRect&) OVERRIDE;
 
-    LocalFrame& frame() const { return *m_frame; }
+    LocalFrame& frame() const
+    {
+        ASSERT(m_frame);
+        return *m_frame;
+    }
+
     Page* page() const;
 
     RenderView* renderView() const;
 
     virtual void setCanHaveScrollbars(bool) OVERRIDE;
 
     virtual PassRefPtr<Scrollbar> createScrollbar(ScrollbarOrientation) OVERRIDE;
 
     virtual void setContentsSize(const IntSize&) OVERRIDE;
     IntPoint clampOffsetAtScale(const IntPoint& offset, float scale) const;
@@ skipping 459 matching lines @@
         m_view->setCanInvalidatePaintDuringPerformLayout(m_originalValue);
     }
 private:
     FrameView* m_view;
     bool m_originalValue;
 };
 
 } // namespace blink
 
 #endif // FrameView_h