Fix ::part() within nesting.

We don't fix the more complex case of :is(:host, #foo) (or the
equivalent rule in nesting), but at least it takes care of
the most common case.

Fixed: 326526716
Change-Id: I6d5e83237964018d06665fd6874166ff68a4bb62
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6496097
Commit-Queue: Steinar H Gunderson <sesse@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1453894}