Correct paint phase for column rules.
We were painting them just before the foreground contents, but they
should be painted just after the multicol border.
There was already a test for scrolling and column rules (which was the
reason why we intentionally did it wrong in the first place) in
fast/multicol/. Convert it into a WPT reftest (column-rule-001.html),
and add a test for correct paint phase (column-rule-002.html).
Bug: 374991969
Change-Id: I5a18f6b6ea02d9baac18a856edd662a7f91f115c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5952468
Reviewed-by: Xianzhu Wang wangxianzhu@chromium.org
Commit-Queue: Morten Stenshorne mstensho@chromium.org
Cr-Commit-Position: refs/heads/main@{#1372958}