MD Downloads: add a use_vulcanize option to Chrome

chrome/chrome_features.gyp

 # Copyright 2015 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.
 
 {
   'includes': [
     'chrome_features.gypi',
   ],
   'targets': [
     {
       # GN version: //chrome/common:features
       'target_name': 'chrome_common_features',
       'includes': [ '../build/buildflag_header.gypi' ],
       'variables': {
         'buildflag_header_path': 'chrome/common/features.h',
         'buildflag_flags': [
           'ENABLE_GOOGLE_NOW=<(enable_google_now)',
           'ANDROID_JAVA_UI=<(android_java_ui)',
+          'USE_VULCANIZE=<(use_vulcanize)',
         ],
       },
     },
   ]
 }