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As a part of message center restyling, this CL adds a new view for
message center when there are no notifier sources.

Previously, there was no such view and the layout was broken when
there were no notifier sources.

TEST=ash_unittests
BUG=726246

Change-Id: I51c6b6b074995003c622e4bd4e1de19d2db5f56b
Reviewed-on: https://chromium-review.googlesource.com/708106
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509688}