[Editor] Add a very basic and incomplete workaround for issue #15780

The main issue is due to the fact that an editor's parent can be null when
we want to serialize it and that lead to an exception which break all the
saving/printing process.
So this incomplete patch fixes only the saving/printing issue but not the
underlying problem (i.e. having a null parent) and doesn't bring that much
complexity, so it should help to uplift it the next Firefox release.

test/integration/test_utils.js

@@ -100,3 +100,21 @@ async function waitForEvent(page, eventName, timeout = 30000) {
   ]);
 }
 exports.waitForEvent = waitForEvent;
+
+const waitForStorageEntries = async (page, nEntries) => {
+  await page.waitForFunction(
+    n => window.PDFViewerApplication.pdfDocument.annotationStorage.size === n,
+    {},
+    nEntries
+  );
+};
+exports.waitForStorageEntries = waitForStorageEntries;
+
+const waitForSelectedEditor = async (page, selector) => {
+  await page.waitForFunction(
+    sel => document.querySelector(sel).classList.contains("selectedEditor"),
+    {},
+    selector
+  );
+};
+exports.waitForSelectedEditor = waitForSelectedEditor;