MD Settings: Lazy load the contents of the "advanced" settings.

chrome/test/data/webui/settings/ensure_lazy_loaded.js

+// Copyright 2017 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.
+
+cr.define('settings', function() {
+  function ensureLazyLoaded() {
+    // Only trigger lazy loading, if we are in top-level Settings page.
+    if (location.href == location.origin + '/') {
+      suiteSetup(function() {
+        return new Promise(function(resolve, reject) {
+          // This URL needs to match the URL passed to <settings-idle-load> from
+          // <settings-basic-page>.
+          Polymer.Base.importHref('/lazy_load.html', resolve, reject, true);
+        });
+      });
+    }
+  }
+
+  return {
+    ensureLazyLoaded: ensureLazyLoaded,
+  };
+});