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

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

Index: third_party/WebKit/LayoutTests/http/tests/fetch/resources/thorough-iframe.html
diff --git a/third_party/WebKit/LayoutTests/http/tests/fetch/resources/thorough-iframe.html b/third_party/WebKit/LayoutTests/http/tests/fetch/resources/thorough-iframe.html
index 30acddc57f02f52c288e363509c59abb7e71caa2..b7296a884b7a490b91f9011d3db28eb03c21462b 100644
--- a/third_party/WebKit/LayoutTests/http/tests/fetch/resources/thorough-iframe.html
+++ b/third_party/WebKit/LayoutTests/http/tests/fetch/resources/thorough-iframe.html
@@ -2,12 +2,22 @@
 <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(