Fix the UK keyboard overlay Enter key shape

chrome/browser/resources/chromeos/keyboard_overlay.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.
  */
 
 body {
   -webkit-user-select: none;
   background: #fff;
   font-size: 100%;
   margin: 0;
@@ skipping 42 matching lines @@
   text-decoration: underline;
   vertical-align: middle;
 }
 
 .keyboard-overlay-learn-more-text a {
   color: #fff;
 }
 
 .keyboard-overlay-key {
   -webkit-box-orient: vertical;
-  background-color: rgba(24, 24, 24, 0.9);
   border: 2px solid #7f7f7f;
   border-radius: 4px;
   color: rgb(151, 151, 151);
   display: -webkit-box;
   font-size: 80%;
   font-weight: bold;
   position: absolute;
 }
 
+.keyboard-overlay-key-background {
+  background-color: rgba(24, 24, 24, 0.9);
+}
+
 .keyboard-overlay-key.is-shortcut {
-  background: -webkit-linear-gradient(rgba(61, 61, 61, 0.8),
-                                      rgba(27, 27, 27, 0.8));
   border-color: #fafafa;
   color: #9e9e9e;
 }
 
+.keyboard-overlay-shortcut-key-background {
+  background: linear-gradient(rgba(61, 61, 61, 0.8), rgba(27, 27, 27, 0.8));
+}
+
 .keyboard-overlay-key.is-shortcut.modifier-shift {
   border-color: rgb(97, 186, 100);
 }
 
 .keyboard-overlay-key.is-shortcut.modifier-ctrl {
   border-color: rgb(93, 128, 199);
 }
 
 .keyboard-overlay-key.is-shortcut.modifier-alt {
   border-color: rgb(184, 84, 84);
@@ skipping 89 matching lines @@
 
 .keyboard-overlay-key-text {
   -webkit-box-ordinal-group: 2;
   padding: 0 3px 2px;
   text-align: center;
 }
 
 .keyboard-overlay-key.is-shortcut .keyboard-overlay-key-text {
   opacity: 0.25;
 }