[cros] Update GAIA frame bg color to match rest of login screen.

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

 /* Copyright (c) 2012 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.
  */
 
 #gaia-signin {
   -webkit-box-align: center;
   -webkit-box-pack: center;
   display: -webkit-box;
   height: 470px;
@@ skipping 27 matching lines @@
   margin-top: 20px;
 }
 
 .signin-link {
   color: #254f9b;
   cursor: pointer;
   text-decoration: none;
 }
 
 #signin-frame {
-  background-color: whitesmoke;
+  background-color: #fefefe;
   border: 1px solid lightgray;
   height: 100%;
   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;
 }
 
 /* Position to be aligned with the login box in iframe  */
 #login-box {
   height: 31px;
   position: absolute;
   top: 77px;
   visibility: hidden;
   z-index: -1;
 }