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.... over chromeos::TabletState::Observer.

This CL is a follow up of crrev.com/c/2491517. It gets rid of
chromeos::TabletState::Observer class in favor of
display::DisplayObserver::OnDisplayTabletStateChanged().

The CL also removes the use of the `friend class` declaration in
chromeos::TabletState: ash::TabletModeController now sets the
tablet state through display::DisplayManager ->
display::DisplayObserver::OnDisplayTabletStateChanged().

Last, the CL deliberately did not got rid of chromeos::TabletState
class. Reason: TabletState can continue to be a central read-only
class for query the tablet state from any where in the source base.

BUG=1113900
R=jamescook@chromium.org

Change-Id: I532e8511c1173139937c36c5e4c282311ea17576
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2518128
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823963}