GitLab

Changes:
1. Make AshColorProvider::Get*LayerColors without |given_color_mode|,
   get color info from IsDarkModeEnabled() instead.
2. Remove AshColorProvider::Get*LayerColorImpl, move all the logic into
   corresponding Get*LayerColors function.
3. Return the color of each type based on IsDarkModeEnabled() now, no
   need to get both light and dark colors and then return on
  |color_mode_|.
4. Oobe and notification don't support dark mode currently. Set the
   colors of them as constant for now. Will switch to AshColorProvider
   later if needed.

Follow up cls:
Remove all the DeprecatedGet*LayerColor and AshColorMode.

Bug: 1107056
Change-Id: Idd3d4ea055ef66cd68c7c1fff356cc313442500d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2376992
Commit-Queue: Min Chen <minch@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804822}