[css-anchor-position] Simple implementation for position-visibility: anchors-valid

Due to the complexity to implement the original proposal of
position-visibility: anchors-valid and anchors-visible in
https://github.com/w3c/csswg-drafts/issues/7758#issuecomment-1965540529,
we have proposed a simple implementation
https://github.com/w3c/csswg-drafts/issues/7758#issuecomment-2026137829
and this implementation is based on the simple proposal.

Bug: 329703412
Change-Id: I7ac5d9bfbd5a3c89822567ffa6701cc66959b665
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5406199


Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280441}