Reviewed by Dan Bernstein.

        REGRESSION (r53110): Find My iPhone message dialog is offset from original position once it gets focus
        https://bugs.webkit.org/show_bug.cgi?id=33821

        Fix an issue with the geometry of fixed position, composited layers inside other
        composited layers. If we called convertToLayerCoords() on a fixed layer with an ancestor
        layer which is not the root, then we assumed that the fixed layer had a positioning root.
        However, this is not always the case, since the fixed layer may have a compositing ancestor
        which is not a fixed container.

        Test: compositing/geometry/fixed-in-composited.html

        * rendering/RenderLayer.cpp:
        (WebCore::isFixedPositionedContainer): Utility function to test whether a layer acts
        as a fixed position container.

        (WebCore::RenderLayer::convertToLayerCoords): When called on a fixed position layer
        for an ancestor layer that is not the root, compute the correct offsets by looking for
        the fixed position container, and computing offsets relative to that. If the ancestorLayer
        is the fixed position container, fall through to the existing code that is shared
        with absolutely positioned layers.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@53742 268f45cc-cd09-0410-ab3c-d52691b4dbfc