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

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

Index: chrome/test/data/webui/settings/settings_idle_render_browsertest.js
diff --git a/chrome/test/data/webui/settings/settings_idle_render_browsertest.js b/chrome/test/data/webui/settings/settings_idle_render_browsertest.js
index 8f3b42138e6c0d2db8aa4c3e0492b48e583710c5..7b8284c26c4309dabf14b7facc8408bed7fd7b19 100644
--- a/chrome/test/data/webui/settings/settings_idle_render_browsertest.js
+++ b/chrome/test/data/webui/settings/settings_idle_render_browsertest.js
@@ -37,9 +37,9 @@ TEST_F('SettingsIdleRenderBrowserTest', 'render', function() {
   suite('Settings idle render tests', function() {
     setup(function() {
       var template =
-          '<template is="settings-idle-render" id="idleTemplate">' +
-          '  <div>' +
-          '  </div>' +
+          '<template is="settings-idle-load" id="idleTemplate" '+
+          '    url="chrome://resources/html/polymer.html">' +

[Dan Beam, 2017/03/18 00:15:51]

why this?

[dpapad, 2017/03/20 20:34:55]

I tried several, this one was the first one that worked (all others threw
various errors in the console). Furthermore, this one makes sense because it
ends up being a no-op (an HTML import for polymer.html is guaranteed to exist
when this test executes, so importHref returns early).

+          '  <div></div>' +
           '</template>';
       document.body.innerHTML = template;
       // The div should not be initially accesible.
@@ -48,9 +48,11 @@ TEST_F('SettingsIdleRenderBrowserTest', 'render', function() {
 
     test('stamps after get()', function() {
       // Calling get() will force stamping without waiting for idle time.
-      var inner = document.getElementById('idleTemplate').get();
-      assertEquals('DIV', inner.nodeName);
-      assertEquals(inner, document.body.querySelector('div'));
+      return document.getElementById('idleTemplate').get().then(
+          function(inner) {
+            assertEquals('DIV', inner.nodeName);
+            assertEquals(inner, document.body.querySelector('div'));
+          });
     });
 
     test('stamps after idle', function(done) {