Add browser tests for FetchEvent.request.mode/credentials in the service worker

chrome/test/data/nacl/pnacl_url_loader/pnacl_url_loader.html

+<script>
+window.addEventListener('load', function() {
+    init();
+  }, false);
+
+function init() {
+  var embed = document.createElement('embed');
+  embed.addEventListener('message', function(message) {
+      window.domAutomationController.setAutomationId(0);
+      window.domAutomationController.send(message.data);
+    }, false);
+  embed.addEventListener('load', function() {
+      embed.postMessage(location.hash.substr(1));
+    }, false);
+  embed.src = 'pnacl_url_loader.nmf';
+  embed.type = 'application/x-pnacl';
+  document.body.appendChild(embed);
+}
+</script>