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This change takes the amount of the border block-start/end that
had been consumed previously when calculating the block size
taken up by the legend. This ensures that adjusted_padding_box_size
does not overcompensate for the border and is consistent across
multiple fragments.

Making this adjustment fixes an associated crash that was introduced to
virtual/layout_ng_block_frag/external/wpt/css/css-break/fieldset-001.html

Bug: 875235
Change-Id: Idcd1c6eba435eac41ca90acd2e4a0cabe4015a3b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2140720Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Commit-Queue: Alison Maher <almaher@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#758289}