Collapse bottom margins of a last child and its parent if parent's height=auto

This patch adds support of the margin collapsing case for the next vertically-adjacent box edges:
- bottom margin of a last in-flow child and bottom margin of its parent if the parent has 'auto' computed height
W3C spec: https://www.w3.org/TR/CSS2/box.html#collapsing-margins

BUG=635619

Review-Url: https://codereview.chromium.org/2336353002
Cr-Commit-Position: refs/heads/master@{#418407}
4 files changed