Calculate minimum column height after layout.

This moves some multicol specific code out from LayoutBlock and
LayoutBlockFlow. More importantly, though, is that once we add optimizations
to skip subtrees during multicol layout, collecting this information during
layout isn't going to work.

R=leviw@chromium.org

Review URL: https://codereview.chromium.org/1406973008

Cr-Commit-Position: refs/heads/master@{#358171}
13 files changed