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This pref will control whether smart dim should be enabled for a user. The
default value is true (i.e. Enabled). The next cl will add policy so that
enterprise users can enforce a different policy.

As this is a power-related pref, it is added to power_prefs[.h|.cc].
However as there is no need to send its value to powerd, it is
intentionally left out of PowerPrefs::UpdatePowerPolicyFromPrefs.

Bug: 862461
Change-Id: Ibaa705362c9951aecae78d9ce28780651067a6ea
Reviewed-on: https://chromium-review.googlesource.com/1166266Reviewed-by: Dan Erat <derat@chromium.org>
Commit-Queue: Jia Meng <jiameng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581435}