[anchor-position] Support logical keywords

This patch supports `start`, `end`, `self-start`, and
`self-end` keywords for the `anchor()` function[1].

[1] https://drafts.csswg.org/css-anchor-1/#anchor-pos

Bug: 1382524
Change-Id: Ie2a1e3bb3653475facac0454461327dd69e8ee89
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4060987


Auto-Submit: Koji Ishii <kojii@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1076083}