cleanup: Don't assume hasOwnProperty() method on objects

Since ES5, it is possible to create objects with no prototype at all:

    let foo = Object.create(null);

Those object won't have any builtin properties like hasOwnProperty(),
which is why eslint added a corresponding rule to its default rule set.

While this isn't an issue that affects our code, there's no harm in fol-
lowing the recommendation and call the method through Object.prototype.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/716

js/ui/sessionMode.js

@@ -111,7 +111,7 @@ function _loadMode(file, info) {
     let suffix = name.indexOf('.json');
     let modeName = suffix == -1 ? name : name.slice(name, suffix);
 
-    if (_modes.hasOwnProperty(modeName))
+    if (Object.prototype.hasOwnProperty.call(_modes, modeName))
         return;
 
     let fileContent, success_, newMode;