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Only the most recent notification will be shown.

Adding a few new components:
 - NotificationMenuController
   - Handles adding/removing the container MenuItemView to an app
     context menu, and passing notification data to
     NotificationMenuView.
 - NotificationMenuView
   - Handles showing NotificationItemViews, showing the proper
     notifications, and in the future showing the overflow area.
 - NotificationItemView
   - Shows basic notification data.
 - NotificationMenuHeaderView
   - Shows "Notifications" text and a counter showing the number
     of notifications.

Please see the design doc and spec, linked in the code review request.

Bug: 801015
Change-Id: I907794db5bcf90e5238773ca7cf9978d539b0855
Reviewed-on: https://chromium-review.googlesource.com/1095431
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568331}