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This is a reland of 59692b72

Change from original CL (PS#1): Fix use-after-free in unit test.

Original change's description:
> Observe ClockModel directly from DateView/TimeView
>
> This CL changes DateView and TimeView to directly observe ClockModel.
> It removes unrelated logic from SystemTrayItem and makes embeding these
> views easier.
>
> TEST=ash_unittests
> BUG=none
>
> Change-Id: I70b3a873c0ac259a05597d3f93b77fd7519dd462
> Reviewed-on: https://chromium-review.googlesource.com/1025615
> Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
> Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#553433}

TBR=stevenjb@chromium.org

Bug: none
Change-Id: I0c609729e5ccc7fea66a696b57a689097f3465af
Reviewed-on: https://chromium-review.googlesource.com/1029390
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553906}