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

LayoutTests/http/tests/serviceworker/ServiceWorkerGlobalScope/resources/scope.js

 function scope_test(t, worker_url, scope) {
   var expectedScope, options;
   if (scope) {
     expectedScope = new URL(scope, document.location).toString();
   } else {
     expectedScope = new URL('/', document.location).toString();
   }
 
   var registration;
   var options = scope ? {scope: scope} : {};
   service_worker_unregister(t, scope)
     .then(function() {
         return navigator.serviceWorker.register(worker_url, options);
       })
     .then(function(r) {
         registration = r;
-        return wait_for_update(t, registration);
-      })
-    .then(function(worker) {
+        var worker = r.installing;
         return new Promise(function(resolve) {
             var messageChannel = new MessageChannel();
             messageChannel.port1.onmessage = resolve;
             worker.postMessage('', [messageChannel.port2]);
           });
       })
     .then(function(e) {
         var message = e.data;
         assert_equals(message.initialScope, expectedScope,
                       'Worker should see the scope on eval.');
         assert_equals(message.currentScope, expectedScope,
                       'Worker scope should not change.');
         return registration.unregister();
       })
     .then(function() {
         t.done();
       })
     .catch(unreached_rejection(t));
 }