ServiceWorker: add ServiceWorkerClients.claim() support (3/3).

LayoutTests/http/tests/serviceworker/resources/claim-worker.js

+self.addEventListener('message', function(event) {
+    self.clients.claim()
+      .then(function(result) {
+          if (result !== undefined) {
+              event.data.port.postMessage(
+                  'FAIL: claim() should be resolved with undefined');
+              return;
+          }
+          event.data.port.postMessage('PASS');
+        })
+      .catch(function(error) {
+          event.data.port.postMessage('FAIL: exception: ' + error.name);
+        });
+  });