[@container] style() query unset on root element crashes
Discovered when trying to fix issue 376834363.
StyleResolver::ComputeValue() does not set up a parent style for the
root element which meant applying unset would crash. There was already a
check for inherit. Do the same for unset.
Bug: 376834363
Change-Id: I075dd462668436f98a6f7924929f5baba6eeba09
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5999996
Commit-Queue: Rune Lillesveen futhark@chromium.org
Reviewed-by: Anders Hartvoll Ruud andruud@chromium.org
Cr-Commit-Position: refs/heads/main@{#1379181}