Convert use_seccomp_bpf to the BUILDFLAG system.

sandbox/linux/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("//build/config/features.gni")
 import("//build/config/nacl/config.gni")
+import("//sandbox/features.gni")
 import("//testing/test.gni")
 
 if (is_android) {
   import("//build/config/android/rules.gni")
 }
 
 declare_args() {
   compile_suid_client = is_linux
 
   compile_credentials = is_linux
@@ skipping 414 matching lines @@
 
 if (is_android) {
   # TODO(GYP_GONE) Delete this after we've converted everything to GN.
   group("sandbox_linux_unittests_deps") {
     testonly = true
     deps = [
       ":sandbox_linux_unittests",
     ]
   }
 }