Look inside inner nested multicols to calculate minimum space shortage.

This is needed in order to make sure that the column balancer gives us as short
outer columns as possible. Otherwise we risk not finding the absolute minimum
space shortage (and thus over-stretch) (or, even worse, not be able to find any
shortage at all).

BUG=447718
R=leviw@chromium.org

Review URL: https://codereview.chromium.org/1471403002

Cr-Commit-Position: refs/heads/master@{#361501}
4 files changed