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

ui/login/display_manager.js

Index: ui/login/display_manager.js
diff --git a/ui/login/display_manager.js b/ui/login/display_manager.js
index b8c6b1d27f278ecf064e87f0fc1f35a5ce7f65a4..b44e359ab3357e0564f710ac1a29261355317238 100644
--- a/ui/login/display_manager.js
+++ b/ui/login/display_manager.js
@@ -919,8 +919,9 @@ cr.define('cr.ui.login', function() {
   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);
   };