Allow generation of a chromium outdirs when enable_extensions=false

extensions/browser/api/vpn_provider/BUILD.gn

 # 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.
 
+import("//extensions/features/features.gni")
+
+assert(enable_extensions,
+       "Cannot depend on extensions because enable_extensions=false.")
+
 assert(is_chromeos)
 
 source_set("vpn_provider") {
   sources = [
     "vpn_provider_api.cc",
     "vpn_provider_api.h",
     "vpn_service.cc",
     "vpn_service.h",
     "vpn_service_factory.h",
   ]
 
   deps = [
     "//extensions/common/api",
   ]
 
   public_deps = [
     "//extensions/browser:browser_sources",
   ]
 }