WebUI Dark Mode: enable by default on Mac

R=thestig@chromium.org
BUG=883049

Change-Id: I2dc81de46b816debcb8e31042c4078e91331e258
Reviewed-on: https://chromium-review.googlesource.com/c/1492977
Commit-Queue: Dan Beam <dbeam@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Auto-Submit: Dan Beam <dbeam@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636285}
1 file changed