Dark mode: implement high contrast

Bug: 850098
Change-Id: I60f0ea58fc989d179227d0c60a3cc150808284f7
Reviewed-on: https://chromium-review.googlesource.com/c/1399506
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621310}
5 files changed