[Xwt] Always call the base methods so widgets are sized correctly

If we do not call base.SetBackendSize then the widget is not added to
the queue of widgets which need resizing. As such, Window.AdjustSize
will never be invoked and top level dialogs will end up completely
ignoring the initial size that is set for them. By invoking the base
methods unconditionally, we get the correct/desired behaviour.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=9203