Do not change the workspace when entering/leaving overview

* This also fixes the IsSelecting so that it returns false
during shutdown.

BUG=834400
TEST=Covered by unit test

Change-Id: Iac0cb8a3bdf03609727c31c51000c52ececfc66a
Reviewed-on: https://chromium-review.googlesource.com/1140749
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576078}
3 files changed