Allow generation of a chromium outdirs when enable_extensions=false

extensions/browser/api/cast_channel/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("cast_channel") {
   sources = [
     "cast_auth_util.cc",
     "cast_auth_util.h",
     "cast_channel_api.cc",
     "cast_channel_api.h",
     "cast_channel_enum_util.cc",
     "cast_channel_enum_util.h",
     "cast_framer.cc",
     "cast_framer.h",
@@ skipping 18 matching lines @@
     "//components/cast_channel",
     "//extensions/common/api",
     "//extensions/common/api/cast_channel:cast_channel_proto",
     "//net",
   ]
 
   public_deps = [
     "//extensions/browser:browser_sources",
   ]
 }