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We need to report unbreakable block-size from the column layout
algorithm, in cases where we have nested column balancing.

Set up the fragment builder to be block-fragmented when inside a nested
multicol that performs the initial column balancing pass, so that
spanners report unbreakability (happens automatically via
NGBoxFragmentBuilder::AddResult(), when set up correctly).

When we fall back to legacy layout, we also need to set up the builder
so that we report unbreakable block-size rather than space shortage
(those two share data storage). We don't actually have to report any
size, though, (0 is fine) since this is always monolithic content, which
is taken care of by the parent algorithm (BreakBeforeChildIfNeeded() in
NGBlockLayoutAlgorithm).

Bug: 829028, 1013716
Change-Id: Ib507cbedbacf2ed05c1db5a58f7738c070f3ab79
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1859786Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705533}