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This CL allows to show the notification with buttons in create_notification
request, and allows Exosphere to handle this request.
Also, this CL adds clicked event for that protocol.

BUG=chromium:838531
TEST=NotificationTest.*, manual verification

Change-Id: Ib7d0693695afac5c732a8c0808a9b3250fa7a327
Reviewed-on: https://chromium-review.googlesource.com/c/1276045
Commit-Queue: Toshiki Kikuchi <toshikikikuchi@google.com>
Reviewed-by: David Reveman <reveman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602213}