Fixed TPM dialog shield style

chrome/browser/resources/chromeos/login/oobe.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.
  *
  * This is the stylesheet used by the Out of the box experience (OOBE) flow.
  */
 
 body {
   background: -webkit-gradient(radial, center center, 0, center center, 400,
               from(#fefefe), to(#efefef));
@@ skipping 87 matching lines @@
 }
 
 .header-section.right {
   left: 50px;
 }
 
 .header-section.left {
   left: -20px;
 }
 
-
 .step {
   -webkit-transition: left .2s, opacity .2s, visibility .2s ease-in-out;
   left: 0;
   min-height: 294px;
   opacity: 1;
   position: absolute;
   width: 640px;
 }
 
 .step.right {
@@ skipping 227 matching lines @@
 }
 
 [hidden] {
   display: none !important;
 }
 
 #popup-overlay {
   -webkit-box-align: center;
   -webkit-box-pack: center;
   -webkit-transition: 0.25s opacity;
-  background: -webkit-radial-gradient(rgba(127, 127, 127, 0.5),
-                                      rgba(127, 127, 127, 0.5) 35%,
-                                      rgba(0, 0, 0, 0.7));
+  background: #fff;
   bottom: 0;
   display: -webkit-box;
   left: 0;
+  opacity: 0.75;
   padding-bottom: 130px;
   padding: 20px;
   position: fixed;
   right: 0;
   top: 0;
   z-index: 5;
 }
 
 #enroll-login-screen {
   display: -webkit-box;
@@ skipping 272 matching lines @@
 #version-labels {
   -webkit-transition: all .5s linear;
   bottom: 10px;
   color: #999;
   font-size: 11px;
   left: 10px;
   opacity: 1.0;
   position: absolute;
   text-shadow: 0 1px 1px #fff;
 }