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

chrome/browser/resources/vr_shell/vr_shell_ui_api.js

 // 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.
 
 var api = {};
 
 /**
  * Enumeration of scene update commands.
  * @enum {number}
  * @const
@@ skipping 609 matching lines @@
    */
   onUpdateTab(tab) {}
 
   /**
    * Remove a tab.
    * @param {Object} tab
    */
   onRemoveTab(tab) {}
 
   /**
+   * Set back/forward history buttons to enabled/disabled.
+   * @param {boolean} canGoBack
+   * @param {boolean} canGoForward
+   */
+  onSetHistoryButtonsEnabled(canGoBack, canGoForward) {}
+
+  /**
    * This function is executed after command parsing completes.
    */
   onCommandHandlerFinished() {}
 
   /** @final */
   handleCommand(dict) {
     if ('mode' in dict) {
       this.onSetMode(dict['mode']);
     }
     if ('fullscreen' in dict) {
@@ skipping 26 matching lines @@
     }
     if ('setTabs' in dict) {
       this.onSetTabs(dict['setTabs']);
     }
     if ('updateTab' in dict) {
       this.onUpdateTab(dict['updateTab']);
     }
     if ('removeTab' in dict) {
       this.onRemoveTab(dict['removeTab']);
     }
+    if ('canGoBack' in dict) {
+      this.onSetHistoryButtonsEnabled(dict['canGoBack'], dict['canGoForward']);
+    }
 
     this.onCommandHandlerFinished()
   }
 };