merge_pr_51

[LayoutNG] Don't allow breaks at the start of a fragmentainer.

We had this almost right, but failed when there was a break with low
appeal inside a child.

Bug: 829028
Change-Id: I67c0d07db85b8de6d9ac3f685263fb1d3b490760
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1940273
Reviewed-by: Koji Ishii kojii@chromium.org
Reviewed-by: Ian Kilpatrick ikilpatrick@chromium.org
Commit-Queue: Morten Stenshorne mstensho@chromium.org
Cr-Commit-Position: refs/heads/master@{#719919}