Add a string for the follow theme option in customization

Bug: chromium:1444588
Change-Id: I1e42dcded66e83c708c8c244fa7238efcf0e4a4d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4754918


Auto-Submit: Sean Kau <skau@chromium.org>
Commit-Queue: Sean Kau <skau@chromium.org>
Commit-Queue: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1180612}