Mac: Add command line switch to force dark mode

Bug: 850098
Change-Id: I8d18484774982a8ec0272f62f0bd77a43668eb0d
Reviewed-on: https://chromium-review.googlesource.com/1238796
Reviewed-by: ccameron <ccameron@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593347}
5 files changed