Bug 1762169 [wpt PR 33420] - Do not traverse a slot node if recalculating slot assign...

Bug 1762169 [wpt PR 33420] - Do not traverse a slot node if recalculating slot assignment, a=testonly

Automatic update from web-platform-tests
Do not traverse a slot node if recalculating slot assignment

ClusterFuzz detected the DCHECK crash caused by the recursive slot's
RecalcAssignment when the children are changed. This CL changes not
to traverse the slot node if the slot assignment is already
recalculating.

Bug: 1309343
Change-Id: I55b44efd9ad2f02f15782a19912553c9907c5dab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3555375
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Mason Freed <masonf@chromium.org>
Commit-Queue: Miyoung Shin <myid.shin@igalia.com>
Cr-Commit-Position: refs/heads/main@{#987316}

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wpt-commits: 15eead7596598ae9231764354e73bb36c678cf6b
wpt-pr: 33420