overview: Add OnDisplayAdded and adjust OnDisplayRemoved

OverviewSession::OnDisplayRemoved is unreachable and contains a TODO
that describes behavior contrary to the current design decisions. Also,
when a display is added, overview ends (as expected) only because of
OverviewSession::OnShelfAlignmentChanged; the present CL adds
OverviewSession::OnDisplayAdded to improve code maintainability.

Test: ash_unittests OverviewSessionTest.Shutdown
Test: ash_unittests OverviewSessionTest.AddDisplay
Test: ash_unittests OverviewSessionTest.RemoveDisplay
Test: ash_unittests OverviewSessionTest.RemoveDisplayWithAnimation
Change-Id: If2a282364b72f5fcedac5bdbb7a9f0c1ab8ebd97
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1743114Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686563}

ash/wm/overview/overview_session.cc

@@ -755,11 +755,16 @@ void OverviewSession::OnHighlightedItemClosed(OverviewItem* item) {
   item->CloseWindow();
 }
 
-void OverviewSession::OnDisplayRemoved(const display::Display& display) {
-  // TODO(flackr): Keep window selection active on remaining displays.
+void OverviewSession::OnDisplayAdded(const display::Display& display) {
   EndOverview();
 }
 
+void OverviewSession::OnDisplayRemoved(const display::Display& display) {
+  // Removing a display causes a window activation which will end overview mode
+  // so that |OnDisplayRemoved| is never called.
+  NOTREACHED();
+}
+
 void OverviewSession::OnDisplayMetricsChanged(const display::Display& display,
                                               uint32_t metrics) {
   // For metrics changes that happen when the split view mode is active, the

ash/wm/overview/overview_session.h

@@ -239,6 +239,7 @@ class ASH_EXPORT OverviewSession : public display::DisplayObserver,
   void OnHighlightedItemClosed(OverviewItem* item);
 
   // display::DisplayObserver:
+  void OnDisplayAdded(const display::Display& display) override;
   void OnDisplayRemoved(const display::Display& display) override;
   void OnDisplayMetricsChanged(const display::Display& display,
                                uint32_t metrics) override;