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

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

Index: LayoutTests/http/tests/serviceworker/resources/claim-worker.js
diff --git a/LayoutTests/http/tests/serviceworker/resources/claim-worker.js b/LayoutTests/http/tests/serviceworker/resources/claim-worker.js
new file mode 100644
index 0000000000000000000000000000000000000000..7cdac8f56c00e71badf520965b87933d43f87f8b
--- /dev/null
+++ b/LayoutTests/http/tests/serviceworker/resources/claim-worker.js
@@ -0,0 +1,9 @@
+self.addEventListener('message', function(event) {
+    self.clients.claim()
+      .then(function() {
+          event.data.port.postMessage('PASS');
+        })
+      .catch(function(error) {
+          event.data.port.postMessage('FAIL: exception: ' + error);
+        });
+  });