Add feature flag to allow dark mode if enabled on system or forced

Bug: 850098
Change-Id: Ib2af48ceb226bbe2650c65331d4bbec2321c0956
Reviewed-on: https://chromium-review.googlesource.com/1251328
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595113}
2 files changed