Ignore background-clip for @page backgrounds.
See https://drafts.csswg.org/css-page-3/#painting
Since the background painting area for the page box is the bleed area,
which covers the entire page box, including its margins, background-clip
should be ignored (treated as border-box).
See also https://www.w3.org/TR/css-backgrounds-3/#background-clip
and how the "root element has a different background painting area".
Bug: 40286153
Change-Id: I45ff3ccd9e1a2b8c1b0759154fc01c132480800a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5828872
Reviewed-by: Fredrik Söderquist fs@opera.com
Commit-Queue: Morten Stenshorne mstensho@chromium.org
Cr-Commit-Position: refs/heads/main@{#1350071}