These appear to have been for ensuring that you
only paint elements in a given subtree. It's not clear
to me exactly how you get to painting an element that
is not rooted at the RenderLayer you started with.
I think it's just not possible in Sky anymore.
This code was added in 2004 for drag images, which
we no longer support.
R=esprehn@chromium.org
Review URL: https://codereview.chromium.org/882223005
bdc24bf049