Don't pass an (out) argument to meta_window_get_outer_rect()

Mutter now has the needed (out) annotation, so we have to use the
return value instead of passing a reference to a rectangle.

js/ui/workspace.js

@@ -881,8 +881,7 @@ Workspace.prototype = {
 
         xCenter = xCenter * global.screen_width;
 
-        let rect = new Meta.Rectangle();
-        metaWindow.get_outer_rect(rect);
+        let rect = metaWindow.get_outer_rect();
         let buttonOuterHeight, captionHeight;
         let buttonOuterWidth = 0;