Adds button to VRShell menu mode, which lets the user manually exit WebVR presentation.

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 55 matching lines @@
   'RELOAD': 2,
   'ZOOM_OUT': 3,
   'ZOOM_IN': 4,
   'RELOAD_UI': 5,
   'LOAD_URL': 6,
   'OMNIBOX_CONTENT': 7,
   'SET_CONTENT_PAUSED': 8,
   'SHOW_TAB': 9,
   'OPEN_NEW_TAB': 10,
   'KEY_EVENT': 11,
+  'EXIT_PRESENT': 12
 };
 
 /**
  * Enumeration of modes that can be specified by the native side.
  * @enum {number}
  * @const
  */
 api.Mode = {
   'UNKNOWN': -1,
   'STANDARD': 0,
@@ skipping 674 matching lines @@
     if ('removeTab' in dict) {
       this.onRemoveTab(dict['removeTab']);
     }
     if ('canGoBack' in dict) {
       this.onSetHistoryButtonsEnabled(dict['canGoBack'], dict['canGoForward']);
     }
 
     this.onCommandHandlerFinished()
   }
 };