Convert use_seccomp_bpf to the BUILDFLAG system.

components/nacl/loader/sandbox_linux/BUILD.gn

 # Copyright 2015 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/ui.gni")
 
 assert(is_linux)
 assert(enable_nacl)
 
 source_set("sandbox_linux") {
   sources = [
     "nacl_bpf_sandbox_linux.cc",
     "nacl_sandbox_linux.cc",
   ]
 
   defines = [ "IN_NACL_HELPER=1" ]
 
   deps = [
     "//base",
     "//components/nacl/common",
     "//components/nacl/loader",
     "//content/public/common",
     "//crypto",
     "//ipc",
     "//sandbox",
+    "//sandbox:sandbox_features",
     "//sandbox/linux:sandbox_services_headers",
   ]
 
   if (use_glib) {
     configs += [ "//build/config/linux:glib" ]
   }
-
-  if (use_seccomp_bpf) {
-    defines += [ "USE_SECCOMP_BPF" ]
-  }
 }