Reviewed by Dan Bernstein.

        Video can overlap position:fixed element when scrolling
        https://bugs.webkit.org/show_bug.cgi?id=32180

        When the scroll position changes, and we're using overlap to decide what gets composited,
        then we need to re-evaluate what gets compositing when scrolling in case fixed postion elements
        overlap composited elements (e.g. video).

        Test: compositing/geometry/video-fixed-scrolling.html

        * page/FrameView.cpp:
        (WebCore::FrameView::updateCompositingLayers):
        (WebCore::FrameView::scrollPositionChanged): Annotate the calls to updateCompositingLayers()
        with the type of change that occurred.

        * rendering/RenderLayer.cpp:
        (WebCore::RenderLayer::updateCompositingAndLayerListsIfNeeded): Ditto.

        * rendering/RenderLayerCompositor.h:
        * rendering/RenderLayerCompositor.cpp:
        (WebCore::RenderLayerCompositor::updateCompositingLayers): For scroll changes, check
        for hierarchy update if m_compositingConsultsOverlap is true, and do
        a geometry update. For other changes, always check for hierarchy updates.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@54006 268f45cc-cd09-0410-ab3c-d52691b4dbfc