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- Add DeprecatedGet*LayerColor functions in a separate file to help
  keeping UI elements' current colors before launching dark/light mode.

- Add ScopedLightModeAsDefeault to set the default color mode to light.
  This is done to support UI elements are light by default. E.g. Power
  button menu.

Bug: 1133952
Change-Id: I169f13991a649a1d1aacea451a6fb8ba8001396c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2440661
Commit-Queue: Min Chen <minch@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813003}