Allow generation of a chromium outdirs when enable_extensions=false

extensions/browser/api/serial/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("serial") {
   sources = [
     "serial_api.cc",
     "serial_api.h",
     "serial_connection.cc",
     "serial_connection.h",
     "serial_event_dispatcher.cc",
     "serial_event_dispatcher.h",
   ]
 
   deps = [
     "//device/serial",
     "//extensions/common/api",
   ]
 
   public_deps = [
     "//extensions/browser:browser_sources",
   ]
 }