Bug 1836565 [wpt PR 40369] - Make ::backdrop stay in top layer while animating out...

Bug 1836565 [wpt PR 40369] - Make ::backdrop stay in top layer while animating out, a=testonly

Automatic update from web-platform-tests
Make ::backdrop stay in top layer while animating out

This patch adds a new internal pseudo class which matches popovers while
they are still in the top layer after they have been closed in order to
also make the corresponding ::backdrop stay in the top layer.

This is based on futhark's patch:
https://chromium-review.googlesource.com/c/chromium/src/+/4554016

HTML spec PR: https://github.com/whatwg/html/pull/9387

Fixed: 1449145
Change-Id: I8e4831e960c5d18fb077f023c119fd0e678541df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4585724
Reviewed-by: Mason Freed <masonf@chromium.org>
Commit-Queue: Joey Arhar <jarhar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1158640}

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wpt-commits: 83976e7d1f6a7b175947209736bd40e94a7cae77
wpt-pr: 40369