[ServiceWorker] Add LayoutTests for the fallback behavior of FetchEvent.

LayoutTests/http/tests/serviceworker/resources/fetch-request-fallback-iframe.html

+<script>
+function xhr(url) {
+  return new Promise(function(resolve, reject) {
+      var request = new XMLHttpRequest();
+      request.addEventListener(
+        'error',
+        function(event) { reject(event); });
+      request.addEventListener(
+        'load',
+        function(event) { resolve(request.response); });
+      request.open('GET', url);
+      request.send();
+    });
+}
+</script>