js: Initialize some properties

Otherwise those can result in the (harmless) "reference to undefined
property" warnings.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/970

js/ui/overview.js

@@ -137,6 +137,7 @@ class OverviewActor extends St.BoxLayout {
 var Overview = class {
     constructor() {
         this._initCalled = false;
+        this._visible = false;
 
         Main.sessionMode.connect('updated', this._sessionUpdated.bind(this));
         this._sessionUpdated();