Make sure to destroy a custom scrollbar's RenderObject's before the arena goes away.  When our widget
        parent is nulled out is a good time.

        Reviewed by Oliver Hunt

        * rendering/RenderScrollbar.cpp:
        (WebCore::RenderScrollbar::~RenderScrollbar):
        (WebCore::RenderScrollbar::setParent):
        (WebCore::RenderScrollbar::updateScrollbarPart):
        * rendering/RenderScrollbar.h:



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@37463 268f45cc-cd09-0410-ab3c-d52691b4dbfc