void RenderScrollbarPart::layoutVerticalPart()
{
    if (m_part == ScrollbarBGPart) {
        computeScrollbarWidth();
        setHeight(m_scrollbar->height());
    } else {
        setWidth(m_scrollbar->width());
        computeScrollbarHeight();
    } 
}
void LayoutScrollbarPart::layoutHorizontalPart()
{
    if (m_part == ScrollbarBGPart) {
        setWidth(m_scrollbar->width());
        computeScrollbarHeight();
    } else {
        computeScrollbarWidth();
        setHeight(m_scrollbar->height());
    }
}