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Also stop observing the MessageCenter, which is in another process (in
--mash).

The UMA is a little wonky, even before this change, because manually
closing the notification is not explicitly logged. It will need to be
reworked in the new style of notifications because if closing doesn't
count as "decline" then nothing does. For now this more or less
maintains existing behavior.

Bug: 783018
Change-Id: I69278c0653d8548ccd1fe6a66015b7cf08a0d08c
Reviewed-on: https://chromium-review.googlesource.com/777739Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517883}