Fix issue with `getTextNodes` and nodes from other iframes

I don't believe this issue will ever occur in the real application, but
we currently have tests (in `html-test.js`) where the passed node comes from
a different iframe and therefore a different JS environment where globals including
`Text` and `Element` have different identities.

src/annotator/anchoring/xpath-util.js

@@ -70,10 +70,12 @@ export function xpathFromNode(node, root) {
 export function getTextNodes(element) {
   const nodes = [];
   for (let node of Array.from(element.childNodes)) {
-    if (node instanceof Text) {
-      nodes.push(node);
-    } else if (node instanceof Element) {
-      nodes.push(...getTextNodes(node));
+    // We test `nodeType` here rather than using `instanceof` because we have
+    // tests where `node` comes from a different iframe.
+    if (node.nodeType === Node.TEXT_NODE) {
+      nodes.push(/** @type {Text} */ (node));
+    } else if (node.nodeType === Node.ELEMENT_NODE) {
+      nodes.push(...getTextNodes(/** @type {Element} */ (node)));
     }
   }
   return nodes;