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By using policy settings, administrator can enforce devices to allow or
disallow notifications from specified URL origins.
The enforced permissions are properly shown on chrome://settings,
however, they were not reflected in notification center settings of
Chrome OS.

This CL changes the view to show disabled icon and make the row dim for
such notifier sources, making it clear for users that they can't change
the setting.

Screenshot: http://screen/gTg2H5OWxaK
UX Mock: https://crbug.com/771269#c11

TEST=manual
BUG=771269

Change-Id: Iee0032ef85a4c132860f15c0f926551810543466
Reviewed-on: https://chromium-review.googlesource.com/810248
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523987}