Dark Mode: add a base::Feature to respond to dark mode in WebUI

Example combined usage CL: https://crrev.com/c/1354685

BUG=883049
R=thestig@chromium.org

Change-Id: Ie6f44e2095a0c59738acec58b478b5a5e22bbd2f
Reviewed-on: https://chromium-review.googlesource.com/c/1357627
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612896}
2 files changed