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

chrome/browser/resources/chromeos/chromevox2/chromevox.gyp

 # 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.
 
 {
   'conditions': [
     ['chromeos==1 and disable_nacl==0 and disable_nacl_untrusted==0', {
+      'variables': {
+        'use_chromevox_next': 1,
+      },
       'includes': [ '../chromevox/common.gypi', ],
       'targets': [
         {
           'target_name': 'chromevox2',
           'type': 'none',
           'dependencies': [
             'chromevox2_copied_scripts',
             'chromevox2_deps',
             'chromevox2_manifest',
             'chromevox2_guest_manifest',
@@ skipping 36 matching lines @@
               'files': [
                 'cvox2/background/background.html',
               ],
             },
           ],
         },
         {
           'target_name': 'chromevox2_manifest',
           'type': 'none',
           'variables': {
-            'output_manifest_path': '<(chromevox_dest_dir)/manifest.json'
+            'output_manifest_path': '<(chromevox_dest_dir)/manifest_next.json'
           },
           'includes': [ '../chromevox/generate_manifest.gypi', ],
         },
         {
           'target_name': 'chromevox2_guest_manifest',
           'type': 'none',
           'variables': {
-            'output_manifest_path': '<(chromevox_dest_dir)/manifest_guest.json',
+            'output_manifest_path': '<(chromevox_dest_dir)/manifest_next_guest.json',
             'is_guest_manifest': 1,
           },
           'includes': [ '../chromevox/generate_manifest.gypi', ],
         },
       ],
     }],
   ],
 }