GitLab

Various changes:
- Start adding DEPS exceptions in preparation for banning
ui/message_center dependencies more broadly.
- Remove a file full of constants in ui/message_center
- delete some unnecessary message_center.h includes
- remove some message center color constants in favor of direct
  use of Label::SetAutoColorReadabilityEnabled.

BUG=783018,723882
TBR=rogerta@chromium.org

Change-Id: Ia7b9332d575a77581691f65f98c945754820c212
Reviewed-on: https://chromium-review.googlesource.com/879501Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534721}