Bug 1632773 [wpt PR 23222] - FindKeyframesRules should use originating element.,...

Bug 1632773 [wpt PR 23222] - FindKeyframesRules should use originating element., a=testonly

Automatic update from web-platform-tests
FindKeyframesRules should use originating element.

It's only using the TreeScope to find @keyframes in which case using the
originating element for pseudo elements should work. Even better when
the originating element is a shadow host, so that we look up :host rules
from the shadow tree for :host::before { animation-name: anim }.

Bug: 961941
Change-Id: I38edec156eaaa7cfb3680762b135323deae00a68
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2162988
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762791}

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wpt-commits: 245ed57a5e400fff2dc4adbf8e452d64397399cd
wpt-pr: 23222