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Previously, SystemClockObserver worked as a model class of system clock,
but it was owned by TraySystemInfo. It has several problems:
* It cannot be referenced from UnifiedSystemTray in the future.
* The model variable is duplicated over multi displays.

This CL
* Renames SystemClockObserver to ClockModel and makes it owned by
  SystemTrayModel.
* Moves AddObserver/RemoveObserver from SystemTrayNotifier
* Moves |hour_clock_type_| model variable from SystemTrayController to
  ClockModel.

TEST=ash_unittests
BUG=none

Change-Id: Ia8d852206b1625b0d23aecaceda5f02d95275207
Reviewed-on: https://chromium-review.googlesource.com/977361
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546424}