[cros] Add support for transparent background in sign in 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 33 matching lines @@
   text-decoration: none;
 }
 
 #signin-frame {
   background-color: #fefefe;
   border: 1px solid lightgray;
   height: 100%;
   width: 340px;
 }
 
+html[oobe=new] #signin-frame {
+  background-color: transparent;
+  background-image: -webkit-linear-gradient(top,
+                                            rgba(255, 255, 255, 0.988281),
+                                            rgba(255, 255, 255, 0.949219));
+}
+
 #gaia-loading {
   -webkit-box-align: center;
   -webkit-box-pack: center;
   bottom: 0;
   display: -webkit-box;
   left: 0;
   position: absolute;
   right: 0;
   top: 0;
 }
 
 #gaia-loading .spinner {
   height: 32px;
   width: 32px;
 }
 
 /* Position to be aligned with the login box in iframe  */
 #login-box {
   height: 31px;
   position: absolute;
   top: 77px;
   visibility: hidden;
   z-index: -1;
 }