Bug 1874681 [wpt PR 43997] - [css-page-3] Support @page margin at-rules, a=testonly

Bug 1874681 [wpt PR 43997] - [css-page-3] Support @page margin at-rules, a=testonly

Automatic update from web-platform-tests
[css-page-3] Support @page margin at-rules (#43997)

Parse subrules of @page that deal with page margin boxes
(e.g.  @top-left).

This is for parsing (and serialization) only; the styles don't actually
do anything yet. It's unclear whether there is a fully defined
serialization spec for these.

We do not currently expose this through CSSOM, in part because it is
unclear what the interface should look like.

Bug: 320370
Fuchsia-Binary-Size: New functionality in the parser
Change-Id: I03e25631758a229ae0124d9fbdd816dfa4316568
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5189281
Commit-Queue: Steinar H Gunderson <sesse@chromium.org>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1249487}

Co-authored-by: Steinar H. Gunderson <sesse@chromium.org>
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wpt-commits: c2c84d06f1c21d463656e1f4a2649081bec8fe9a
wpt-pr: 43997