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This CL is a preparation CL, where the final goal is to move
away from using chromeos::TabletState, embedding its functionality
into //display/ScreenBase class.

The first step to to dismantle this enum from it, so that it can
be referenced to independently.

TBR=jamescook@chromium.org

BUG=1113900
R=jamescook@chromium.org, oshima@chromium.org, rjkroege@chromium.org

Change-Id: I4d907b9d322723f581bfc1a278577088028795ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2505944
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822105}