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This class will get more logic to show / hide notifications so
extracting it into a separate file so we can test it easily. Also moves
construction into the NotificationDisplayService so we can handle
dependencies there.

Bug: 1131375
Change-Id: I7693c18f4e2f8e677b9eb4c40079dfde802140f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2452649Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Richard Knoll <knollr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815594}