overview: Hide the overview on session mode hasOverview changes

If the sessionMode does not allow to show the overview, we should also
hide an already visible overview.

This fixes a bug where, if the lockscreen was shown while the overview
was visible, the Ctrl+Alt+Tab popup would allow navigating inside the
overview because the overview actor is still mapped.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/1043

js/ui/overview.js

@@ -199,7 +199,11 @@ var Overview = class {
     }
 
     _sessionUpdated() {
-        this.isDummy = !Main.sessionMode.hasOverview;
+        const { hasOverview } = Main.sessionMode;
+        if (!hasOverview)
+            this.hide();
+
+        this.isDummy = !hasOverview;
         this._createOverview();
     }