Move layer() into RenderBox and make it non-virtual.  Move all of the scrolling functions from RenderObject
        to RenderBox and devirtualize as many as possible.  Clean up styleWillChange to factor code specific to
        RenderBox and RenderBlock into their appropriate subclasses.  Stubbed out an empty styleWillChange on
        RenderText, since none of the work done in styleWillChange was relevant to text nodes.

        Reviewed by Sam Weinig

        * dom/Document.cpp:
        (WebCore::Document::elementFromPoint):
        (WebCore::Document::renderView):
        (WebCore::Document::prepareMouseEvent):
        * dom/Document.h:
        * dom/Element.cpp:
        (WebCore::Element::scrollByUnits):
        * editing/SelectionController.cpp:
        (WebCore::SelectionController::contains):
        * html/HTMLMarqueeElement.cpp:
        (WebCore::HTMLMarqueeElement::start):
        (WebCore::HTMLMarqueeElement::stop):
        * page/Accessibility...