Update test to cover
        <rdar://problem/5521068>
        Visibility not propagated correctly for children of a layer with z-index
        
        The fix for this was (accidentally) already checked in with r27277, 
        with retroactive r=hyatt.

        * fast/layers/layer-visibility.html:
        * platform/mac/fast/layers/layer-visibility-expected.checksum:
        * platform/mac/fast/layers/layer-visibility-expected.png:
        * platform/mac/fast/layers/layer-visibility-expected.txt:



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@27489 268f45cc-cd09-0410-ab3c-d52691b4dbfc