Allow generation of a chromium outdirs when enable_extensions=false

extensions/browser/guest_view/web_view/web_ui/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.")
+
 source_set("web_ui") {
   sources = [
     "web_ui_url_fetcher.cc",
     "web_ui_url_fetcher.h",
   ]
   public_deps = [
     "//base",
     "//net",
     "//url",
   ]
 }