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

ui/login/display_manager.js

 // Copyright 2014 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.
 
 /**
  * @fileoverview Display manager for WebUI OOBE and login.
  */
 
 // TODO(xiyuan): Find a better to share those constants.
 /** @const */ var SCREEN_OOBE_NETWORK = 'connect';
@@ skipping 901 matching lines @@
   };
 
   /**
    * Resets sign-in input fields.
    * @param {boolean} forceOnline Whether online sign-in should be forced.
    *     If |forceOnline| is false previously used sign-in type will be used.
    */
   DisplayManager.resetSigninUI = function(forceOnline) {
     var currentScreenId = Oobe.getInstance().currentScreen.id;
 
-    $(SCREEN_GAIA_SIGNIN).reset(
-        currentScreenId == SCREEN_GAIA_SIGNIN, forceOnline);
+    if ($(SCREEN_GAIA_SIGNIN))
+      $(SCREEN_GAIA_SIGNIN).reset(
+          currentScreenId == SCREEN_GAIA_SIGNIN, forceOnline);
     $('login-header-bar').disabled = false;
     $('pod-row').reset(currentScreenId == SCREEN_ACCOUNT_PICKER);
   };
 
   /**
    * Shows sign-in error bubble.
    * @param {number} loginAttempts Number of login attemps tried.
    * @param {string} message Error message to show.
    * @param {string} link Text to use for help link.
    * @param {number} helpId Help topic Id associated with help link.
@@ skipping 109 matching lines @@
    */
   DisplayManager.refocusCurrentPod = function() {
     $('pod-row').refocusCurrentPod();
   };
 
   // Export
   return {
     DisplayManager: DisplayManager
   };
 });