[LayoutNG] WIP on storing layout output in the legacy tree for multicol.

Painting NG fragment trees is still not supported, so in the meantime
we'll keep LayoutMultiColumnFlowThread and LayoutMultiColumnSet, and have
multicol painted and hit-tested in the old-fashioned way (just like
everything else, really).

Based on work by glebl.

BUG=635619

Review-Url: https://codereview.chromium.org/2786923004
Cr-Commit-Position: refs/heads/master@{#461409}
2 files changed