Reviewed by Eric Seidel

        Expose an isRootLayer() method on RenderLayer that works whether the
        layer is rooted or not, and use that in a few places in the
        accelerated compositing code.

        * rendering/RenderLayer.h:
        (WebCore::RenderLayer::isRootLayer):
        * rendering/RenderLayerBacking.cpp:
        (WebCore::RenderLayerBacking::paintingGoesToWindow):
        * rendering/RenderLayerCompositor.cpp:
        (WebCore::RenderLayerCompositor::rebuildCompositingLayerTree):
        (WebCore::RenderLayerCompositor::requiresCompositingLayer):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@40731 268f45cc-cd09-0410-ab3c-d52691b4dbfc