Export getLayers and setLayers methods of ol.layer.Group

This is nicer than having to use .get("layers") and .set("layers", x).

src/ol/layer/layergroup.js

@@ -148,6 +148,7 @@ ol.layer.Group.prototype.handleLayersRemove_ = function(collectionEvent) {
  * @return {ol.Collection.<ol.layer.Base>|undefined} Collection of
  * {@link ol.layer.Layer layers} that are part of this group.
  * @observable
+ * @api
  */
 ol.layer.Group.prototype.getLayers = function() {
   return /** @type {ol.Collection.<ol.layer.Base>|undefined} */ (this.get(
@@ -163,6 +164,7 @@ goog.exportProperty(
  * @param {ol.Collection.<ol.layer.Base>|undefined} layers Collection of
  * {@link ol.layer.Layer layers} that are part of this group.
  * @observable
+ * @api
  */
 ol.layer.Group.prototype.setLayers = function(layers) {
   this.set(ol.layer.GroupProperty.LAYERS, layers);