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

LayoutTests/http/tests/serviceworker/postmessage-to-client.html

Index: LayoutTests/http/tests/serviceworker/postmessage-to-client.html
diff --git a/LayoutTests/http/tests/serviceworker/postmessage-to-client.html b/LayoutTests/http/tests/serviceworker/postmessage-to-client.html
index 2f55dcc6cff86c85983b79d6e672ee94b1f6f33f..3c456d67e3e6781e11cf1787eca5c3393e57a938 100644
--- a/LayoutTests/http/tests/serviceworker/postmessage-to-client.html
+++ b/LayoutTests/http/tests/serviceworker/postmessage-to-client.html
@@ -10,10 +10,7 @@ t.step(function() {
     service_worker_unregister_and_register(
         t, 'resources/postmessage-to-client-worker.js', scope)
       .then(function(registration) {
-          return wait_for_update(t, registration);
-        })
-      .then(function(sw) {
-          return wait_for_state(t, sw, 'activated');
+          return wait_for_state(t, registration.installing, 'activated');
         })
       .then(function() { return with_iframe(scope); })
       .then(function(frame) {