Reviewed by Dan Bernstein.

        Compositing layers that are clipped out by an overflow:scroll parent fail to get created on reveal
        https://bugs.webkit.org/show_bug.cgi?id=38712

        When compositing layer creation is testing layer overlap, we need to re-run the
        algorithm when an overflow:scroll element scrolls, to create new layers for revealed
        elements.

        Test: compositing/layer-creation/overflow-scroll-overlap.html

        * rendering/RenderLayer.cpp:
        (WebCore::RenderLayer::scrollToOffset): First, move the compositing updates
        until after the call to updateWidgetPositions(), so that this order is similar
        to what we done when a FrameView scrolls. This change has no known side effects.
        Second, if compositingConsultsOverlap() is true, we need to actually do a
        compositing layer update to compute whether revealed/hidden layers should
        be created/destroyed.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@58936 268f45cc-cd09-0410-ab3c-d52691b4dbfc