Allow generation of a chromium outdirs when enable_extensions=false

extensions/common/api/cast_channel/BUILD.gn

 # Copyright 2014 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")
 import("//third_party/protobuf/proto_library.gni")
 
+assert(enable_extensions,
+       "Cannot depend on extensions because enable_extensions=false.")
+
 proto_library("cast_channel_proto") {
   sources = [
     "authority_keys.proto",
     "cast_channel.proto",
     "logging.proto",
   ]
 }