overview: Test that overview mode ends when a display is added

The present CL adds a unit test OverviewSessionTest.AddDisplay to verify
that overview mode ends when a display is added.

Bug: 988839
Change-Id: I529f45bb5120bf03affcb07b0075050c296ee25e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1743854
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685260}
1 file changed