Rework to register Document and add XMLSerializer

src/ol/xml.js

@@ -486,3 +486,27 @@ export function pushSerializeAndPop(object, serializersNS, nodeFactory, values,
   serialize(serializersNS, nodeFactory, values, objectStack, opt_keys, opt_this);
   return /** @type {O|undefined} */ (objectStack.pop());
 }
+
+let xmlSerializer_ = undefined;
+
+/**
+ * Register a XMLSerializer. Can be used
+ * to inject a XMLSerializer where there is no globally available implementation.
+ *
+ * @param {XMLSerializer} xmlSerializer A XMLSerializer.
+ * @api
+ */
+export function registerXMLSerializer(xmlSerializer) {
+  xmlSerializer_ = xmlSerializer;
+}
+
+/**
+ * Get a document that should be used when creating nodes for XML serializations.
+ * @return {XMLSerializer} The XMLSerializer.
+ */
+export function getXMLSerializer() {
+  if (xmlSerializer_ === undefined && typeof XMLSerializer !== 'undefined') {
+    xmlSerializer_ = new XMLSerializer();
+  }
+  return xmlSerializer_;
+}