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

LayoutTests/http/tests/serviceworker/fetch-request-resources.html

Index: LayoutTests/http/tests/serviceworker/fetch-request-resources.html
diff --git a/LayoutTests/http/tests/serviceworker/fetch-request-resources.html b/LayoutTests/http/tests/serviceworker/fetch-request-resources.html
index 89d6f08df14456fd5aabe7b6d3d0e39b46bbbb4c..8ca323b880e0f655fe24b5bd9859e9dc754056c7 100644
--- a/LayoutTests/http/tests/serviceworker/fetch-request-resources.html
+++ b/LayoutTests/http/tests/serviceworker/fetch-request-resources.html
@@ -92,11 +92,8 @@ async_test(function(t) {
     var frame;
     service_worker_unregister_and_register(t, SCRIPT, SCOPE)
       .then(function(registration) {
-          return wait_for_update(t, registration);
-        })
-      .then(function(sw) {
-          worker = sw;
-          return wait_for_state(t, sw, 'activated');
+          worker = registration.installing;
+          return wait_for_state(t, worker, 'activated');
         })
       .then(function() {
           return new Promise(function(resolve) {