Clean up VR Shell mode transitions (and fix potential webvr startup race).

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 = (function() {
   'use strict';
 
   /**
    * Enumeration of scene update commands.
    * @enum {number}
@@ skipping 65 matching lines @@
 
   /**
    * Enumeration of modes that can be specified by the native side.
    * @enum {number}
    * @const
    */
   var Mode = Object.freeze({
     'UNKNOWN': -1,
     'STANDARD': 0,
     'WEB_VR': 1,
-    'MENU': 2,
-    'CINEMA': 3
   });
 
   /**
    * Triggers an Action.
    */
   function doAction(action) {
     chrome.send('doAction', [action]);
   }
 
   /**
@@ skipping 205 matching lines @@
     Action: Action,
     Mode: Mode,
     getContentElementId: getContentElementId,
     UiElement: UiElement,
     UiElementUpdate: UiElementUpdate,
     Animation: Animation,
     doAction: doAction,
     domLoaded: domLoaded,
   };
 })();