Service Worker: Add a test for calling respondWith() asynchronously.

LayoutTests/http/tests/serviceworker/resources/fetch-event-async-respond-with-worker.js

+var result;
+
+self.addEventListener('message', function(event) {
+    event.data.port.postMessage(result);
+  });
+
+self.addEventListener('fetch', function(event) {
+    setTimeout(function() {
+        try {
+          event.respondWith(new Response());
+          result = 'FAIL: did not throw';
+        } catch (error) {
+          if (error.name == 'InvalidStateError')
+            result = 'PASS';
+          else
+            result = 'FAIL: Unexpected exception: ' + error;
+        }
+      }, 0);
+  });