Remove margin collapsing.

Margin collapsing is a document-oriented feature. For applications
it just causes confusion and slowness.

margins.sky has failure output because offsetTop is returning the
wrong values. That's not new with this patch though. When I look
at it visually in skydb, everything seems to be in the right place.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/700743002