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Only support native notifications, i.e. those passed over mojo.

Some tests are permanently removed on Chrome OS. Some tests are
migrated to message_center_unittests.

Bug: 578868
Change-Id: Ibd4edcd1968c94af2d24b15ed3c1329ea7bf49b1
Reviewed-on: https://chromium-review.googlesource.com/1083937Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564928}