Add notifications to UKM.xml.

Added notifications and related metrics to ukm.xml and included logging
of some metrics that have been recorded to the database already,
including the number of clicks, number of action button clicks and the
ClosedReason for the notification.

UKM Collection Review:
https://docs.google.com/document/d/1oUfHs_t5gEOhMYvjoC2so2Ie_Pt51m12XIgEpbzfU1k/edit?usp=sharing

Bug: 842622
Change-Id: Iabe9e3065e5d0c5c677545873d2e1ea630aea45d
Reviewed-on: https://chromium-review.googlesource.com/1071649
Commit-Queue: Sharon Yang <yangsharon@google.com>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Robert Kaplow (OOO until 0730) <rkaplow@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Martin Šrámek <msramek@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Anita Woodruff <awdf@chromium.org>
Reviewed-by: Christian Dullweber <dullweber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578048}
13 files changed