Bug 1837502 [wpt PR 40459] - [anchor-position] Implement auto anchor fallbacks, a...

Bug 1837502 [wpt PR 40459] - [anchor-position] Implement auto anchor fallbacks, a=testonly

Automatic update from web-platform-tests
[anchor-position] Implement auto anchor fallbacks

This patch implements auto anchor fallbacks by introducing an
`OOFCandidateStyleIterator` class that encapsulates the iteration of
all candidate styles for an OOF, including the base style, `@try`
fallback styles and automatic fallback styles. This class also
replaces the existing position fallback style enumeration.

The existing selectmenu UA stylesheet is also simplified using
auto anchor fallbacks.

Spec: https://drafts.csswg.org/css-anchor-position-1/#fallback-automatic

Fixed: 1412864
Change-Id: I2931001ed255afd611343a7f40e7334cf823cd10
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4600528
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Mason Freed <masonf@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1157079}

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wpt-commits: c2363480357cc2fd52ca4d90d20b3d9af29767fd
wpt-pr: 40459