Bug 1812693 [wpt PR 38204] - Convert animation-timeline property value to tree-scoped...

Bug 1812693 [wpt PR 38204] - Convert animation-timeline property value to tree-scoped, a=testonly

Automatic update from web-platform-tests
Convert animation-timeline property value to tree-scoped

Following the plan in crrev.com/c/4167268, this patch converts the
animation-timeline property to use tree-scoped-populated CSS values
instead of ScopedCSSValue.

There's a minor behavior change that the support of <string> values in
the property has been removed. This is because:
- The spec doesn't allow <string> values [1]
- Allowing <string> values will complicate the implementation, as it
  requires populating TreeScope into a CSSStringValue, which doesn't
  make much sense

Note: animation-timeline is currently an experimental feature, so we
don't need any Blink intents to modify its behavior.

[1] https://drafts.csswg.org/css-animations-2/#propdef-animation-timeline

Bug: 1395026
Change-Id: I8a44e8d2759a49368cbe864d8c3285aa0a79dca3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4196352
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Auto-Submit: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1097818}

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wpt-commits: 5ef3eda5c7ca71a2eadc01d9854893c33317ca0e
wpt-pr: 38204