Bug 1492849 - Update the PDF viewer mochitests to utilize the, recently implemented, general re-dispatching of internal events to the DOM. r=bdahl

By updating the PDF viewer mochitests to instead use the new support for re-dispatching general `EventBus` events to the DOM, as implemented in https://github.com/mozilla/pdf.js/pull/10019, this will allow (future) removal of the hard-coded DOM event dispatching currently present in the https://github.com/mozilla/pdf.js/blob/master/web/dom_events.js file.

browser/extensions/pdfjs/test/browser_pdfjs_navigation.js

@@ -242,9 +242,9 @@ async function runTests(browser) {
 
       // Add an event-listener to wait for page to change, afterwards resolve the promise
       let timeout = window.setTimeout(() => deferred.reject(), 5000);
-      window.addEventListener("pagechange", function pageChange() {
+      window.addEventListener("pagechanging", function pageChange() {
         if (pageNumber.value == test.expectedPage) {
-          window.removeEventListener("pagechange", pageChange);
+          window.removeEventListener("pagechanging", pageChange);
           window.clearTimeout(timeout);
           deferred.resolve(+pageNumber.value);
         }

browser/extensions/pdfjs/test/head.js

@@ -1,11 +1,16 @@
-function waitForPdfJS(browser, url) {
+async function waitForPdfJS(browser, url) {
+  await SpecialPowers.pushPrefEnv({
+    set: [
+      ["pdfjs.eventBusDispatchToDOM", true],
+    ],
+  });
   // Runs tests after all "load" event handlers have fired off
   return ContentTask.spawn(browser, url, async function(contentUrl) {
     await new Promise((resolve) => {
       // NB: Add the listener to the global object so that we receive the
       // event fired from the new window.
-      addEventListener("documentload", function listener() {
-        removeEventListener("documentload", listener, false);
+      addEventListener("documentloaded", function listener() {
+        removeEventListener("documentloaded", listener, false);
         resolve();
       }, false, true);