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Shield widget is getting removed. This is step one by moving the cannot
snap window out into its own separate widget owned by overview_session.
Since there is only ever one shown, it doesn't need to be owned by
overviewGrid.

Test: ash_unittests OverviewSessionTest.*
Bug: 942759
Change-Id: I5c9e40763a35d27f2e383effada0f4f6d8791320
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1535131Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644538}