Back button for the WebView-based sign-in flow.

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 {
   height: 609px; /* Should be the same as #user-image.loading min-heigth. */
   padding: 70px 17px 69px;    /* Screen has no controls. */
   width: 722px; /* Should be the same as #user-image.loading width. */
 }
@@ skipping 55 matching lines @@
 }
 
 .loading #close-button-item {
   background-image: url(chrome://theme/IDR_LOGIN_CLOSE_BUTTTON_ON_WHITE);
 }
 
 #close-button-item:disabled {
   background-image: url(chrome://theme/IDR_LOGIN_CLOSE_BUTTTON_MASKED);
 }
 
+#back-button-item {
+  background: transparent none;
+  background-image: url(chrome://theme/IDR_LOGIN_BACK_BUTTTON);
+  background-position: center;
+  background-repeat: no-repeat;
+  height: 18px;
+  left: 14px;
+  position: absolute;
+  top: 14px;
+  width: 18px;
+  z-index: 1;
+}
+
+html[dir=rtl] #close-button-item {
+  left: auto;
+  right: 14px;
+}
+
+#back-button-item:disabled {
+  background-image: url(chrome://theme/IDR_LOGIN_BACK_BUTTTON_MASKED);
+}
+
 .signin-text {
   color: #666;
   margin-top: 20px;
 }
 
 .signin-link {
   color: rgb(37, 79, 155);
   cursor: pointer;
   text-decoration: none;
 }
@@ skipping 110 matching lines @@
   left: 0;
   position: absolute;
   right: 0;
   text-align: center;
   top: 50px;
 }
 
 #saml-notice-message {
   margin: 0 auto;
 }