Refactor oobe/login screens so lock screen can be loaded faster.

ui/login/account_picker/user_pod_template.js

+// Copyright (c) 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+(function() {
+  "use strict";
+
+  var START_LOADING_DELAY = 1000;
+
+  function doLazyLoad() {
+    function lazyLoadUrl(url) {
+      var link = document.createElement('link');
+      link.rel = 'import';
+      link.href = url;
+      document.body.appendChild(link);
+    }
+
+    lazyLoadUrl('chrome://resources/polymer/v1_0/iron-icons/iron-icons.html');
+  }
+
+  window.addEventListener('load', function() {
+    setTimeout(doLazyLoad, START_LOADING_DELAY);
+  });
+})();