[LayoutNG] Take overflow into account when column-balancing.

I didn't check why one test started to pass (it's not a very interesting
test for overflow).

Bug: 829028
Change-Id: I2c2d5fe3ea8c6a87f00df47f81d4158936d8a1bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1742162
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685349}