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* Add a simple mechanism to keep track of the active shelf item
  in the model
* Use the existing shelf window watcher to update the active shelf
  item
* When rendering shelf items in ShelfView, query the active shelf
  item to show the proper indicator
* Remove the STATUS_ACTIVE value since this is now all managed within
  ash

Bug: 874181
Bug: 805612
Change-Id: I089587646208ee03776f95a4f479c5b6d35d82f8
Reviewed-on: https://chromium-review.googlesource.com/1180274Reviewed-by: Martin Barbella <mbarbella@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585340}