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Hide both for the item currently dragged. Only hide the close button for
the rest of the items.
Spec: https://mccanny.users.x20web.corp.google.com/www/splitscreen-motion/index.html
Window selection and deselection

Test: ash_unittests WindowSelectorTest.WindowItemTitleCloseVisibilityOnDrag
Bug: 782320, 775655
Change-Id: I7e06a0154a9afdb391966109517b1aed419083e4
Reviewed-on: https://chromium-review.googlesource.com/893212Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534764}