Show spinner for constrained signin flow

chrome/browser/resources/inline_login/inline_login.css

 /* Copyright 2013 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.
  */
 
 html,
 body,
 #contents,
 #signin-frame {
   height: 100%;
   margin: 0;
   overflow: hidden;
   padding: 0;
   width: 100%;
 }
 
 #signin-frame,
 #spinner-container {
+  background-color: #f5f5f5;
   bottom: 0;
   left: 0;
   position: absolute;
   right: 0;
   top: 0;
 }
 
 #spinner-container {
   -webkit-box-align: center;
   -webkit-box-pack: center;
   display: -webkit-box;
 }
 
 #contents.loading #signin-frame,
 #contents:not(.loading) #spinner-container {
   display: none;
 }