Visually disable VR Shell back/forward buttons when no history is available

chrome/browser/resources/vr_shell/vr_shell_ui.css

 /* Copyright 2016 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. */
 
 html {
   background-color: rgba(255, 255, 255, 0);
 }
 
 #ui {
   left: 0;
@@ skipping 100 matching lines @@
   text-transform: uppercase;
   vertical-align: middle;
   white-space: nowrap;
   width: 300px;
 }
 
 .rect-button:hover {
   opacity: 1;
 }
 
+.disabled-button {
+  background-color: #aaa;
+}
+
 #back-button,
 #forward-button {
   background-image: url(../../../../ui/webui/resources/images/vr_back.svg);
 }
 
 #reload-button {
   background-image: url(../../../../ui/webui/resources/images/vr_reload.svg);
 }
 
 #forward-button {
@@ skipping 171 matching lines @@
   outline: none; /* Do not show an outline when focused. */
   overflow: hidden;
   white-space: nowrap;
   width: 100%;
 }
 
 #omnibox-clear-button {
   background: url(../../../../ui/webui/resources/images/x-hover.png) no-repeat center center;
   width: 18px;
 }