Add LayoutTest for ServiceWorker's sandbox iframe handling.

LayoutTests/http/tests/serviceworker/chromium/resources/sandboxed-iframe-fetch-event-iframe.html

+<script>
+function with_iframe(url) {
+  return new Promise(function(resolve) {
+      var frame = document.createElement('iframe');
+      frame.src = url;
+      frame.onload = function() { resolve(frame); };
+      document.body.appendChild(frame);
+    });
+}
+
+function with_sandboxed_iframe(url, sandbox) {
+  return new Promise(function(resolve) {
+      var frame = document.createElement('iframe');
+      frame.sandbox = sandbox;
+      frame.src = url;
+      frame.onload = function() { resolve(frame); };
+      document.body.appendChild(frame);
+    });
+}
+
+window.onmessage = function (e) {
+  var id = e.data['id'];
+  fetch(location.href + "_fetch", {mode: 'no-cors'})
+    .then(function() {
+        return with_iframe(location.href + "_iframe");
+      })
+    .then(function() {
+        return with_sandboxed_iframe(location.href + "_script",
+                                     "allow-scripts");
+      })
+    .then(function() {
+        return with_sandboxed_iframe(location.href + "_script-origin",
+                                     "allow-scripts allow-same-origin");
+      })
+    .then(function() {
+        window.top.postMessage({id: id, result: 'done'}, '*');
+      })
+    .catch(function(e) {
+        window.top.postMessage({id: id, result: 'error: ' + e.toString()}, '*');
+      });
+};
+</script>