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Add a NotificationItemView::NotificationItemViewDelegate, which:
 - Activates notifications.
 - Acts as the SlideOutController::Delegate.
 - Allows for easier testing of NotificationMenuView.

Make NotificationMenuController a NotificationItemViewDelegate.

Pass NotificationMenuController AppMenuModelAdapter:
 - Allows NotificationMenuController to close the menu.

Also added tests.

Bug: 843761
Change-Id: I20474838a0fb5360196a3d0585b7e84620b86e9f
Reviewed-on: https://chromium-review.googlesource.com/1120842
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571993}