Convert use_seccomp_bpf to the BUILDFLAG system.

sandbox/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/buildflag_header.gni")
+import("//sandbox/features.gni")
+
 # Meta-target that forwards to the proper platform one.
 group("sandbox") {
   if (is_win) {
     public_deps = [
       "//sandbox/win:sandbox",
     ]
   } else if (is_mac) {
     public_deps = [
       "//sandbox/mac:sandbox",
       "//sandbox/mac:seatbelt",
     ]
   } else if (is_linux || is_android) {
     public_deps = [
       "//sandbox/linux:sandbox",
     ]
   }
 }
+
+buildflag_header("sandbox_features") {
+  header = "sandbox_features.h"
+  flags = [ "USE_SECCOMP_BPF=$use_seccomp_bpf" ]
+}