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* Add a couple shelf layout tests to make sure when Assistant
  is invoked in tablet mode with/without apps, the hotseat state
  won't change.

Bug: None
Test: unit tests
Change-Id: I7103fe9489f780cd441942801aa4d03b71825dbe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2013752
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734728}