[css-flexbox] Fix under-invalidation with fieldset.

This was the classic - use an invalid intrinsic block-size for cache
logic.

Bug: 1270850
Change-Id: Id02cd79b1832ac8f0de0085444fdd8fea6dba5db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3373726


Reviewed-by: David Grogan <dgrogan@chromium.org>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/main@{#956766}