Reviewed by Dan Bernstein.

        Compositing layers not correctly updated after partial layout
        https://bugs.webkit.org/show_bug.cgi?id=30425

        When a partial layout happened, for example as the result of a postion-change-only
        layout, then some compositing layer positions were not correctly updated. To fix
        this, updateLayerPositions() now carries along a flag that is set at the rootmost
        layer being updated, and used to determine when we hit the first compositing layer
        in this update. RenderLayerBacking::updateAfterLayout() makes use of this information
        to do a full geometry update on that layer, which is thus the rootmost compositing
        layer that is being updated.

        Test: compositing/geometry/partial-layout-update.html

        * page/FrameView.cpp:
        (WebCore::FrameView::layout):
        * rendering/RenderLayer.cpp:
        (WebCore::RenderLayer::updateLayerPositions):
        (WebCore::RenderLayer::scrollToOffset):
        * rendering/RenderLayer.h:
        (WebCore::RenderLayer::):
        * rendering/RenderLayerBacking.cpp:
        (WebCore::RenderLayerBacking::updateAfterLayout):
        * rendering/RenderLayerBacking.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@49753 268f45cc-cd09-0410-ab3c-d52691b4dbfc