Reviewed by Dave Hyatt.

        Layer content misplaced with compositing, z-index and overflow
        https://bugs.webkit.org/show_bug.cgi?id=28361

        The foreground GraphicsLayer (used to render element foreground when the element has
        negative z-order children) was misplaced when there is a clipping layer (which clips
        children). The foreground layer is parented under the clipping layer so that the foreground
        depth-sorts with the layer's children, so its geometry needs to be computed relative to
        that clipping layer.

        Also clarified some FIXME comments, and corrected a debug-only layer name.

        Test: compositing/geometry/clipping-foreground.html

        * platform/graphics/mac/GraphicsLayerCA.mm:
        (WebCore::GraphicsLayerCA::updateSublayerList):
        * rendering/RenderLayerBacking.cpp:
        (WebCore::RenderLayerBacking::updateGraphicsLayerGeometry):
        (WebCore::RenderLayerBacking::updateForegroundLayer):
        * rendering/RenderLayerCompositor.cpp:
        (WebCore::RenderLayerCompositor::rebuildCompositingLayerTree):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@47409 268f45cc-cd09-0410-ab3c-d52691b4dbfc