[LayoutNG] Abspos directly after a column spanner

If an abspos element was placed directly after a spanner, a break
before was created to continue layout after the spanner. However, the
containing block for that abspos may be elsewhere. This causes a
problem in NGFragmentChildIterator since the break before will not
match the children that actually get laid out after the column spanner.

This CL fixes this by skipping over break tokens that are the result of
a break before an out-of-flow positioned element in
NGFragmentChildIterator.

The added web test no longer hits the related DCHECK. However, it
appears that the static position is incorrectly calculated. A
separate bug has been opened to investigate: crbug.com/1188436.

Bug: 1176359
Change-Id: Ida02ac5fa3c3b45f4aeae006af38a0e6727e5027
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2762620
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Commit-Queue: Alison Maher <almaher@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#864681}