Reviewed by Dan Bernstein.

        Fix positioning error when a compositing, absolutely-positioned element has an ancestor with opacity.
        https://bugs.webkit.org/show_bug.cgi?id=28754

        For absolutely-positioned layers, convertToLayerCoords() jumps to the positioned
        ancestor, since the layer x and y are relative to that ancestor. However, this could
        skip over the ancestorLayer, thus giving the wrong result.

        Fix by duplicating the enclosingPositionedAncestor() logic in convertToLayerCoords(),
        and checking for ancestorLayer along the way. If found, compute offset of both to
        the enclosingPositionedAncestor() and subtract.

        This also fixes a positioning bug with abs. positioned elements in reflections,
        so there is a new reflection test with a pixel result.

        Tests: compositing/geometry/abs-position-inside-opacity.html
               fast/reflections/abs-position-in-reflection.html

        * rendering/RenderLayer.cpp:
        (WebCore::isPositionedContainer):
        (WebCore::RenderLayer::enclosingPositionedAncestor):
        (WebCore::RenderLayer::enclosingTransformedAncestor):
        (WebCore::RenderLayer::convertToLayerCoords):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@47900 268f45cc-cd09-0410-ab3c-d52691b4dbfc