Add padding:1px to <td> and <th> outside tables, per spec

Per spec:
  https://html.spec.whatwg.org/#tables-2
the UA stylesheet should have "td, th { padding: 1px; }". The
stylesheet addition is only applied when the td/th are outside
a <table> element, since padding is taken care of by the table
code in the normal case.

This behavior is tested via this test:
  external/wpt/css/css-tables/html-to-css-mapping-1.html

Fixed: 1406929
Change-Id: I3fd6ce439569a2425dbeddf9ad0d1e74a4d642ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4200857
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Auto-Submit: Mason Freed <masonf@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1098902}