ServiceWorker cleanup: remove wait_for_update() in favor of r.installing in most tests

LayoutTests/http/tests/local/serviceworker/resources/fetch-request-body-file-iframe.html

 <script>
 var port = undefined;
 var scope = 'fetch-request-body-file-test';
 var script = 'fetch-request-body-file-worker.js';
 
 window.addEventListener('message', function(evt) {
     port = evt.ports[0];
     start();
   }, false);
 
-function wait_for_update(registration) {
+function wait_for_activated(registration) {
   return new Promise(function(resolve) {
-      registration.addEventListener('updatefound', function() {
-          resolve(registration.installing);
-        });
-    });
-}
-
-function wait_for_activated(worker) {
-  return new Promise(function(resolve) {
+      var worker = registration.installing;
       worker.addEventListener('statechange', function() {
           if (worker.state == 'activated') {
             resolve(worker);
           }
         });
     });
 }
 
 function start() {
   if (location.search == '?register') {
     navigator.serviceWorker.register(script, {scope: scope})
       .then(function(registration) {
           return registration.unregister();
         })
       .then(function() {
           return navigator.serviceWorker.register(script, {scope: scope});
         })
-      .then(wait_for_update)
       .then(wait_for_activated)
       .then(function(worker) {
           port.postMessage({msg: 'registered'});
         });
   } else if (location.search == '?unregister') {
     navigator.serviceWorker.register(script, {scope: scope})
       .then(function(registration) {
           return registration.unregister();
         })
       .then(function() {
           port.postMessage({msg: 'unregistered'});
         });
   }
 }
 </script>