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

LayoutTests/http/tests/serviceworker/fetch-body-mixin.html

Index: LayoutTests/http/tests/serviceworker/fetch-body-mixin.html
diff --git a/LayoutTests/http/tests/serviceworker/fetch-body-mixin.html b/LayoutTests/http/tests/serviceworker/fetch-body-mixin.html
index 36d81e8a5c1ecdcbc1694c80e119951595fe0be3..3e5896a5d8facb7b266bd05227cd4a9547d2626c 100644
--- a/LayoutTests/http/tests/serviceworker/fetch-body-mixin.html
+++ b/LayoutTests/http/tests/serviceworker/fetch-body-mixin.html
@@ -9,9 +9,7 @@ async_test(function(t) {
     service_worker_unregister_and_register(
         t, 'resources/fetch-body-mixin-worker.js', scope)
       .then(function(registration) {
-          return wait_for_update(t, registration);
-        })
-      .then(function(sw) {
+          var sw = registration.installing;
           var messageChannel = new MessageChannel();
           messageChannel.port1.onmessage = t.step_func(onMessage);
           sw.postMessage({port: messageChannel.port2}, [messageChannel.port2]);