Simplify PDF viewer initialization in the dev server

Remove logic that is no longer required with the current Hypothesis
client and PDF.js version.

See https://github.com/hypothesis/browser-extension/pull/658.

dev-server/static/scripts/pdfjs-init.js

 'use strict';
 
-// Note: This file is not transpiled.
+/* global PDFViewerApplication, PDFViewerApplicationOptions */
+
 // Listen for `webviewerloaded` event to configure the viewer after its files
 // have been loaded but before it is initialized.
 document.addEventListener('webviewerloaded', () => {
-  const appOptions = window.PDFViewerApplicationOptions;
   const app = window.PDFViewerApplication;
 
-  // Ensure that PDF.js viewer events such as "documentloaded" are dispatched
-  // to the DOM. The client relies on this.
-  appOptions.set('eventBusDispatchToDOM', true);
-
-  // Disable preferences support, as otherwise this will result in `eventBusDispatchToDOM`
-  // being overridden with the default value of `false`.
-  appOptions.set('disablePreferences', true);
-
   // Prevent loading of default viewer PDF.
-  appOptions.set('defaultUrl', '');
+  PDFViewerApplicationOptions.set('defaultUrl', '');
 
   // Read configuration rendered into template as global vars.
   const documentUrl = window.DOCUMENT_URL;
   const url = window.PDF_URL;
   const clientEmbedUrl = window.CLIENT_URL;
 
-  // Wait for the PDF viewer to be fully initialized and then load the Hypothesis client.
-  //
-  // This is required because the client currently assumes that `PDFViewerApplication`
-  // is fully initialized when it loads. Note that "fully initialized" only means
-  // that the PDF viewer application's components have been initialized. The
-  // PDF itself will still be loading, and the client will wait for that to
-  // complete before fetching annotations.
-  //
-  const pdfjsInitialized = new Promise(resolve => {
-    // Poll `app.initialized` as there doesn't appear to be an event that
-    // we can listen to.
-    const timer = setInterval(function () {
-      if (app.initialized) {
-        clearTimeout(timer);
-        resolve();
-      }
-    }, 20);
-  });
-
-  pdfjsInitialized.then(() => {
-    // Prevent PDF.js' `Promise` polyfill, if it was used, from being
-    // overwritten by the one that ships with Hypothesis (both from core-js).
-    //
-    // See https://github.com/hypothesis/via/issues/81#issuecomment-531121534
-    if (
-      typeof Promise === 'function' &&
-      typeof PromiseRejectionEvent === 'undefined'
-    ) {
-      window.PromiseRejectionEvent = function FakePromiseRejectionEvent() {
-        // core-js doesn't actually use this, it just tests for `typeof PromiseRejectionEvent`
-        console.warn('Tried to construct fake `PromiseRejectionEvent`');
-      };
-    }
-
+  PDFViewerApplication.initializedPromise.then(() => {
     // Load the Hypothesis client.
     const embedScript = document.createElement('script');
     embedScript.src = clientEmbedUrl;
@@ -72,7 +31,7 @@ document.addEventListener('webviewerloaded', () => {
     // and https://github.com/mozilla/pdf.js/issues/10435#issuecomment-452706770
     app.open({
       // Load PDF through Via to work around CORS restrictions.
-      url: url,
+      url,
 
       // Make sure `PDFViewerApplication.url` returns the original URL, as this
       // is the URL associated with annotations.