ServiceWorker: Wait for the completion of async tasks on ExtendableMessageEvent

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

 self.addEventListener('message', function(event) {
-    self.clients.claim()
+    event.waitUntil(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);
-        });
+        }));
   });