BUGFIX: ComboBox.addItem should able added separator.

When compiling with Closure Compiler, it warning that `goog.ui.ComboBox`
can not allow codes like that:

```JavaScript
var combobox = new goog.ui.ComboBox();
combobox.addItem(new goog.ui.MenuSeparator());
```

Codes like that should works, but compiler can not passed. So I modified
the annotation and make it works.