(as well as NotificationHandler::ShouldDisplayOnFullScreen) in favor of
an enum in RichNotificationData.

Behaviorally, there's a slight change: switching into fullscreen will
hide a notification even if that notification came from the same
app/website. I think it's more important to support showing
notifications on top of existing fullscreen windows (as appropriate),
so this change doesn't seem overly worrisome.

In the future this enum will also let us get rid of the centralized list
of Ash system notifiers and instead let each Ash system notification
control its own visibility for fullscreen/login screen/etc.

Bug: 776187
Change-Id: I295cd9b18a157b8ba6bbfa27a89f97c9fc6ece85
Reviewed-on: https://chromium-review.googlesource.com/745040


Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513823}