spec: serviceWorker registration with file scheme

spec/fixtures/pages/service-worker/service-worker.js

@@ -0,0 +1,9 @@
+self.addEventListener('fetch', function(event) {
+  var requestUrl = new URL(event.request.url);
+
+  if (requestUrl.pathname === '/echo' &&
+      event.request.headers.has('X-Mock-Response')) {
+    var mockResponse = new Response('Hello from serviceWorker!');
+    event.respondWith(mockResponse);
+  }
+});