[ChromeOS] Show a loading UI before Gaia is loaded for WebUI login.

chrome/browser/resources/chromeos/login/screen_signin.css

 /* Copyright (c) 2011 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.
  */
 
 #signin {
   -webkit-box-align: center;
   -webkit-box-pack: center;
   display: -webkit-box;
   overflow: hidden;
 }
 
 #gaia-signin {
   height: 470px;
 }
 
 #signin-right {
   float: right;
   font-size: 12px;
   margin-right: 80px;
   width: 200px;
 }
 
 .signin-link {
   color: #254f9b;
   cursor: pointer;
   margin-top: 20px;
 }
 
-#gaia-signin > iframe {
+#signin-frame {
   bottom: 0;
-  display: -webkit-box;
   height: 100%;
   left: 0;
   position: absolute;
   right: 0;
   top: 0;
   width: 340px;
 }
 
+#gaia-loading {
+  -webkit-box-align: center;
+  -webkit-box-pack: center;
+  bottom: 0;
+  display: -webkit-box;
+  left: 0;
+  position: absolute;
+  right: 0;
+  top: 0;
+}
+
 #email,
 #password {
   display: block;
   margin-bottom: 20px;
 }