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Before this, the MessageCenterObserver was used to allow the ShelfModel
to keep track of existing notifications. This allowed the ShelfModel to
correctly update the apps that had a notification.

In this CL, the ShelfController no longer observes the MessageCenter and
instead observes the AppRegistryCache to get notification updates. With
this method, the ShelfModel no longer needs to keep a map of existing
notifications. Instead, the AppRegistryCache calls OnAppUpdate()
whenever an app's notification badge status has changed.

Also, in order to get the initial notification badge status for new
ShelfItems, the ShelfControlelr observes changes to the ShelfModel.
When a new item is added to the model, the ShelfController uses the
AppRegistryCache to update the model with the current notification
badge status.

Bug: 1080827
Change-Id: I1c37626170387a8e81cec0f6f3c06026ac62a976
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2219164Reviewed-by: Nancy Wang <nancylingwang@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Matthew Mourgos <mmourgos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786304}