Bug 1760616 [wpt PR 33237] - [FlexNG] Forced break detection, a=testonly

Bug 1760616 [wpt PR 33237] - [FlexNG] Forced break detection, a=testonly

Automatic update from web-platform-tests
[FlexNG] Forced break detection

We perform extra logic for forced breaks to properly handle top
margins. However, we previously looked at HasForcedBreak(),
which is also true if a descendant has a forced break.

We only want to perform this logic if the current child break was
forced. LastChildBreakToken() was added to detect this.

I also noticed that we may consider an item that had previously
broken before to have container separation because its
previous sibling had overflow. Update the container
separation logic for columns to always check if the current
item previously broke.

Bug: 660611
Change-Id: I08ae49dd88062afb0637f8f83be42879f774adab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3533844
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Commit-Queue: Alison Maher <almaher@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#983330}

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wpt-commits: 07201da519dd326fd7a94960cde8487a2cede780
wpt-pr: 33237