Add method to query system dark mode status in NativeTheme

Bug: 850098, 893598
Change-Id: I0ec85e12ae0714974e4d8122914afe330ac0fe93
Reviewed-on: https://chromium-review.googlesource.com/c/1240172
Reviewed-by: Avi Drissman <avi@chromium.org>
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@{#600008}
6 files changed