MD History: Lazily load element files which are not needed for first paint

chrome/browser/resources/md_history/app.js

 // Copyright 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.
 
+cr.define('md_history', function() {
+  var lazyLoadPromise = null;
+  function ensureLazyLoaded() {
+    if (!lazyLoadPromise) {
+      lazyLoadPromise = new Promise(function(resolve, reject) {
+        Polymer.Base.importHref(
+            'chrome://history/lazy_load.html', resolve, reject, true);
+      });
+    }
+    return lazyLoadPromise;
+  }
+
+  return {
+    ensureLazyLoaded: ensureLazyLoaded,
+  };
+});
+
 Polymer({
   is: 'history-app',
 
   behaviors: [Polymer.IronScrollTargetBehavior],
 
   properties: {
     // Used to display notices for profile sign-in status.
     showSidebarFooter: Boolean,
 
     hasSyncedResults: Boolean,
@@ skipping 104 matching lines @@
       chrome.send(
           'metricsHandler:recordTime',
           ['History.ResultsRenderedTime', window.performance.now()]);
     });
 
     // Focus the search field on load. Done here to ensure the history page
     // is rendered before we try to take focus.
     if (!this.hasDrawer_) {
       this.focusToolbarSearchField();
     }
+
+    // Lazily load the remainder of the UI.
+    md_history.ensureLazyLoaded();
   },
 
   /** Overridden from IronScrollTargetBehavior */
   _scrollHandler: function() {
     this.toolbarShadow_ = this.scrollTarget.scrollTop != 0;
   },
 
   /** @private */
   onMenuTap_: function() {
     var drawer = this.$$('#drawer');
@@ skipping 233 matching lines @@
             break;
         }
         break;
     }
 
     md_history.BrowserService.getInstance().recordHistogram(
       'History.HistoryPageView', histogramValue, HistoryPageViewHistogram.END
     );
   },
 });