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Add the logic for syncable dark/light switcher. Enable/Disable the dark
mode through the feature pod in system tray will update the preference
for this user on other devices too.
Login screen will be done in a follow up cl.

Bug: 1106012
Change-Id: Idbbc3f5652effdc7590713fc5687e031e3d97dac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2373134
Commit-Queue: Min Chen <minch@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802737}