Enable runtime switching between ChromeVox and ChromeVox next via command line.

chrome/browser/resources/chromeos/chromevox/generate_manifest.gypi

 # Copyright 2014 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.
 
 # Generates an output manifest based on a Jinja2 templated manifest.
 # Include this file inside of your target to generate a manifest.
 # The following variables must be set before including this file:
 #
 # template_manifest_path: a valid Jinja2 file path.
 # output_manifest_path: file path for the resulting manifest.
 #
-# The following variable is optional:
+# The following variables are optional:
 #
 # guest_manifest: 1 or 0; generates a manifest usable while in guest
 # mode.
+# use_chromevox_next: 1 or 0; generates a manifest for ChromeVox next.
 
 {
   'variables': {
     'generate_manifest_script_path': 'tools/generate_manifest.py',
     'is_guest_manifest%': 0,
     'key': 'MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDltVl1k15pjRzuZfMc3B69inxwm2bZeZ2O8/zFO+NluHnBm3GJ3fzdOoFGJd+M16I8p7zxxQyHeDMfWYASyCeB8XnUEDKjqNLQfCnncsANzHsYoEbYj2nEUML2P13b9q+AAvpCBpAJ4cZp81e9n1y/vbSXHE4385cgkKueItzikQIDAQAB',
+    'use_chromevox_next%': 0,
   },
   'includes': [
     '../../../../../build/util/version.gypi',
   ],
   'actions': [
     {
       'action_name': 'generate_manifest',
       'message': 'Generate manifest for <(_target_name)',
       'inputs': [
         '<(generate_manifest_script_path)',
         '<(template_manifest_path)',
       ],
       'outputs': [
         '<(output_manifest_path)'
       ],
       'action': [
         'python',
         '<(generate_manifest_script_path)',
         '--is_guest_manifest=<(is_guest_manifest)',
         '--key=<(key)',
         '--use_chromevox_next=<(use_chromevox_next)',
         '--set_version=<(version_full)',
         '-o', '<(output_manifest_path)',
         '<(template_manifest_path)',
       ],
     },
   ],
 }