[autofill] Pin preview font-family to a system font

Bug: 916838
Change-Id: I4e874105262f2e15a11a7a18a7afd204e5827400
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1423109
Reviewed-by: Fabio Tirelo <ftirelo@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Roger McFarlane <rogerm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#640884}