This is a dependency of WebNotificationTray, which is a dependency of
StatusAreaWidget, which I am trying to migrate to wm common types.

* Add auto-hide and work area methods to WmShelf.
* Access shelf_widget() via shelf_layout_manager() in WmShelfAura, as we need
to access it before the |Shelf| is created during external monitor init.

BUG=619636
TEST=ash_unittests

Review-Url: https://codereview.chromium.org/2029323002
Cr-Commit-Position: refs/heads/master@{#400312}