Make callback work for old-style Menu.popup call

While the old-style Menu.popup has been deprecated, it is still being
used widely in apps and even in Electron itself.

lib/browser/api/menu.js

@@ -56,6 +56,11 @@ Menu.prototype.popup = function (window, x, y, positioningItem) {
     [newPosition, newY, newX, newWindow] = [y, x, window, null]
   }
 
+  // menu.popup([w], x, y, callback)
+  if (typeof newPosition === 'function') {
+    callback = newPosition
+  }
+
   // menu.popup({})
   if (window != null && window.constructor === Object) {
     opts = window