cleanup: Use object shorthand where possible

ES6 allows to omit property names where they match the name of the
assigned variable, which makes code less redunant and thus cleaner.
We will soon enforce that in our eslint rules, so make sure we use
the shorthand wherever possible.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/805

js/ui/background.js

@@ -444,7 +444,7 @@ var Background = GObject.registerClass({
 
     _loadAnimation(file) {
         this._cache.getAnimation({
-            file: file,
+            file,
             settingsSchema: this._settings.schema_id,
             onLoaded: animation => {
                 this._animation = animation;
@@ -592,11 +592,11 @@ var BackgroundSource = class BackgroundSource {
 
         if (!(monitorIndex in this._backgrounds)) {
             let background = new Background({
-                monitorIndex: monitorIndex,
+                monitorIndex,
                 layoutManager: this._layoutManager,
                 settings: this._settings,
-                file: file,
-                style: style
+                file,
+                style
             });
 
             background._changedId = background.connect('bg-changed', () => {