Handle stale m_owner in FrameView::needsScrollbarReconstruction.

third_party/WebKit/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
@@ skipping 961 matching lines @@
   // Returns true if the FrameView's own scrollbars overlay its content when
   // visible.
   bool hasOverlayScrollbars() const;
 
   // Returns true if the frame should use custom scrollbars. If true, sets
   // customScrollbarElement to the element that supplies the scrollbar's style
   // information.
   bool shouldUseCustomScrollbars(Element*& customScrollbarElement) const;
 
   // Returns true if a scrollbar needs to go from native -> custom or vice
-  // versa.
+  // versa, or if a custom scrollbar has a stale owner.
   bool needsScrollbarReconstruction() const;
 
   bool shouldIgnoreOverflowHidden() const;
 
   void updateScrollCorner();
 
   AXObjectCache* axObjectCache() const;
 
   void setLayoutSizeInternal(const IntSize&);
 
@@ skipping 248 matching lines @@
                   widget.isFrameView());
 DEFINE_TYPE_CASTS(FrameView,
                   ScrollableArea,
                   scrollableArea,
                   scrollableArea->isFrameView(),
                   scrollableArea.isFrameView());
 
 }  // namespace blink
 
 #endif  // FrameView_h