[css-multicol] Mark some functions/classes final

R=mstensho@chromium.org

Change-Id: Ic60362dee1a92a029c470ee06ac3f007f3f08142
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1524904
Auto-Submit: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641141}
3 files changed