Fixed TPM dialog shield style

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

Index: chrome/browser/resources/chromeos/login/oobe.css
diff --git a/chrome/browser/resources/chromeos/login/oobe.css b/chrome/browser/resources/chromeos/login/oobe.css
index 6c9f70f62255743d1f1965e305ad51b13b6bbbc2..ba05921aff29f0faed0c7abaf18f698215819b8d 100644
--- a/chrome/browser/resources/chromeos/login/oobe.css
+++ b/chrome/browser/resources/chromeos/login/oobe.css
@@ -105,7 +105,6 @@ hr.bottom {
   left: -20px;
 }
 
-
 .step {
   -webkit-transition: left .2s, opacity .2s, visibility .2s ease-in-out;
   left: 0;
@@ -353,12 +352,11 @@ button {
   -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;