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This is (a drive by CL) part of effort to reduce the dependency of
/caption_buttons/ on other parts of //ash/public/cpp/. It also
removes some trailing tablet_mode.h includes from //chrome as well.

BUG=1113900
R=sky@chromium.org
TBR=alemate@chromium.org

Change-Id: Ied7a7395ce7ad103fd3ba42282dd55ca1913cc60
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2431727
Commit-Queue: Antonio Gomes (GMT-4) <tonikitoo@igalia.com>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811056}