Allow banners to trigger on sites which don't register a service worker onload.

chrome/test/data/banners/main.js

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 function initialize() {
   navigator.serviceWorker.register('service_worker.js');
 
   window.addEventListener('appinstalled', () => {
     window.document.title = 'Got appinstalled';
   });
 }
 
 function addManifestLinkTag() {
   var url = window.location.href;
   var manifestIndex = url.indexOf("?manifest=");
   var manifestUrl =
       (manifestIndex >= 0) ? url.slice(manifestIndex + 10) : 'manifest.json';
   var linkTag = document.createElement("link");
   linkTag.rel = "manifest";
   linkTag.href = manifestUrl;
   document.head.append(linkTag);
 }
+
+function getSwDelay() {
+  // Allow the test page to set a custom delay (required since desktop is
+  // faster than Android and hence needs a longer delay).
+  var url = window.location.href;
+  var swDelayIndex = url.indexOf("?swdelay=");
+  return (swDelayIndex >= 0) ? url.slice(swDelayIndex + 9) : 300;
+}