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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}