Bug 1768224 [wpt PR 33967] - Calculate legend block-size contribution consistently...

Bug 1768224 [wpt PR 33967] - Calculate legend block-size contribution consistently., a=testonly

Automatic update from web-platform-tests
Calculate legend block-size contribution consistently.

For fragmentation, if we were resuming layout after a break, we used to
calculate it based on the consumed block-size stored in the break token
of the fieldset itself and the break token of the anonymous content
wrapper child. Turns out it's too hard to get this right. For instance,
if the contents wrapper breaks before the fragmentation line, due to
tall trailing padding, we'd get this wrong (and count the "pagination
strut" as part of the legend block-size contribution).

So here's a solution to that: store the block-size contribution in the
break token.

Bug: 1318137
Change-Id: I01b66477bf79f1076da897da8e76d4080410450a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3629328
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Alison Maher <almaher@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1000694}

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wpt-commits: 6c9f7ead6913fb4de763dbd620b290bd86e2cc4c
wpt-pr: 33967