Initial implementation of Collapsing Margins computational logic for LayoutNG

This patches introduces the logic that computes Collapsing Margins for the
following pairs:
- top margin of a box and top margin of its first in-flow child
- bottom margin of box and top margin of its next in-flow following sibling

This implementation doesn't take into account any Margins Collapsing style exceptions, e.g. padding, borders etc.

Design document: https://docs.google.com/document/d/1uxbDh4uONFQOiGuiumlJBLGgO4KDWB8ZEkp7Rd47fw4/edit#bookmark=id.gk14n6z7jt8a

BUG=635619

Review-Url: https://codereview.chromium.org/2313873002
Cr-Commit-Position: refs/heads/master@{#417293}
15 files changed