ServiceWorker: Add support for .skipWaiting and controllerchange event(3/3)

LayoutTests/http/tests/serviceworker/resources/skip-waiting-worker.js

+importScripts('worker-testharness.js');
+
+promise_test(function() {
+    return self.skipWaiting()
+      .then(function(result) {
+          assert_equals(result, undefined,
+                        'Promise should be resolved with undefined');
+        })
+      .then(function() {
+          var promises = [];
+          for (var i = 0; i < 8; ++i)
+            promises.push(skipWaiting());
+          return Promise.all(promises);
+        })
+      .then(function(results) {
+          results.forEach(function(r) {
+              assert_equals(r, undefined,
+                            'Promises should be resolved with undefined');
+            });
+        });
+  }, 'skipWaiting');