ServiceWorker: Add an API to fallback to renderer for CORS preflight

third_party/WebKit/LayoutTests/http/tests/fetch/resources/thorough-iframe.html

 <script src = "fetch-test-options.js"></script>
 <script src = "thorough-util.js"></script>
 <script>
 function onMessage(event) {
-  var script = document.createElement('script');
-  script.src = event.data.url;
-  script.addEventListener(
-    'error',
-    function() { report({jsonpResult:'error'}); });
-  document.body.appendChild(script);
+  if (event.data.fetch) {
+    var request = new Request(event.data.url,
+                              {credentials: 'same-origin', mode: 'no-cors'});
+    doFetch(request)
+      .then(function(message) {
+          message.response = undefined;
+          report(message);
+        });
+  } else if (event.data.url) {
+    var script = document.createElement('script');
+    script.src = event.data.url;
+    script.addEventListener(
+      'error',
+      function() { report({jsonpResult:'error'}); });
+    document.body.appendChild(script);
+  }
 }
 function report(data) {
   window.parent.postMessage(
     data,
     BASE_ORIGIN);
 }
 function onload() {
   window.addEventListener('message', onMessage, false);
 }
 window.addEventListener('load', onload);
 </script>