[LayoutNG] Nested block fragmentation support in multicol.

No attempt has yet been made to handle column balancing correctly in nested
fragmentation situations. Still, this CL makes one such test pass.

Testing needs to be done using unit tests, since there's no support for
painting fragmented blocks on the painting side yet.

Bug: 829028
Change-Id: I449c2e0c931100064e453d97c78bc64b11760824
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1566174
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653973}
5 files changed