code: function CartoDBSubLayer(layer, position) {
    SubLayerBase.call(this, layer, position);
    this._bindInteraction();
  
    var layer = this._parent.getLayer(this._position);
    // TODO: Test this
    if (Backbone.Model && layer) {
      this.infowindow = new Backbone.Model(layer.infowindow);
      this.infowindow.bind('change', function() {
        layer.infowindow = this.infowindow.toJSON();
        this._parent.setLayer(this._position, layer);
      }, this);
    }
  }
docs: CartoDB / Mapnik sublayers