Remove margin collapsing.

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

Committed: https://chromium.googlesource.com/external/mojo/+/cc308be13b0c58a6f7fa39c879b8a067aaadc508

[ojan, 2014-11-04 04:55:40]

ojan@chromium.org changed reviewers:
+ abarth@chromium.org, eseidel@chromium.org, esprehn@chromium.org

[esprehn, 2014-11-04 04:59:30]

lgtm

[ojan, 2014-11-04 05:30:47]

Committed patchset #1 (id:1) manually as
cc308be13b0c58a6f7fa39c879b8a067aaadc508 (presubmit successful).