Add default @page margin box content alignment to UA stylesheet.
See https://drafts.csswg.org/css-page-3/#margin-text-alignment
This doesn't really provide sensible defaults for vertical writing
modes, though. See w3c/csswg-drafts#10421
Bug: 40341678
Change-Id: Icf0093cc675a09fb2c941bc5c235bdaec467f62d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5642035
Reviewed-by: Rune Lillesveen futhark@chromium.org
Commit-Queue: Morten Stenshorne mstensho@chromium.org
Cr-Commit-Position: refs/heads/main@{#1317000}