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This used to fail a DCHECK when nested multicol was specified on a
fieldset at an outer fragmentainer boundary, but apart from that, it
worked fine, because BreakBeforeChildIfNeeded() correctly detects the
situation (abortion) and inserts a break before. Just move the DCHECK a
bit down.

No code changes are required for legends, as they are monolithic. Just
replace the TODO with a proper comment, and keep the DCHECK (the legend
test also passes without the code changes in this CL).

Bug: 829028
Change-Id: I62bf97f9bd04c185f86e93103f204cc6d872c438
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2595288Reviewed-by: Alison Maher <almaher@microsoft.com>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837755}