Allow generation of a chromium outdirs when enable_extensions=false

extensions/browser/api/usb/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("usb") {
   sources = [
     "usb_api.cc",
     "usb_api.h",
     "usb_device_resource.cc",
     "usb_device_resource.h",
     "usb_event_router.cc",
     "usb_event_router.h",
     "usb_guid_map.cc",
     "usb_guid_map.h",
   ]
 
   configs += [
     # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
     "//build/config/compiler:no_size_t_to_int_warning",
   ]
 
   deps = [
     "//content/public/browser",
     "//content/public/common",
     "//device/usb",
     "//extensions/common/api",
   ]
 
   public_deps = [
     "//extensions/browser:browser_sources",
   ]
 }