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/+/1743854Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685260}

ash/wm/overview/overview_session_unittest.cc

@@ -1216,6 +1216,15 @@ TEST_F(OverviewSessionTest, Shutdown) {
   ToggleOverview();
 }
 
+// Tests adding a display during overview.
+TEST_F(OverviewSessionTest, AddDisplay) {
+  UpdateDisplay("400x400");
+  ToggleOverview();
+  EXPECT_TRUE(InOverviewSession());
+  UpdateDisplay("400x400,400x400");
+  EXPECT_FALSE(InOverviewSession());
+}
+
 // Tests removing a display during overview.
 TEST_F(OverviewSessionTest, RemoveDisplay) {
   UpdateDisplay("400x400,400x400");