Upstream service worker `fetch` test to WPT

third_party/WebKit/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();
-    });
-}
-
-function load_image(url, cross_origin) {
-  return new Promise(function(resolve, reject) {
-      var img = document.createElement('img');
-      document.body.appendChild(img);
-      img.onload = function() {
-        resolve();
-      };
-      img.onerror = function() {
-        reject();
-      };
-      if (cross_origin != '') {
-        img.crossOrigin = cross_origin;
-      }
-      img.src = url;
-    });
-}
-</script>