Convert use_seccomp_bpf to the BUILDFLAG system.

sandbox/BUILD.gn

Index: sandbox/BUILD.gn
diff --git a/sandbox/BUILD.gn b/sandbox/BUILD.gn
index 8ca3574e182b3b1422ac4a67a8ffeabb2be196df..8c0405ed8ec9565863b7e813bf0686a46bc42340 100644
--- a/sandbox/BUILD.gn
+++ b/sandbox/BUILD.gn
@@ -2,6 +2,9 @@
 # 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) {
@@ -19,3 +22,8 @@ group("sandbox") {
     ]
   }
 }
+
+buildflag_header("sandbox_features") {
+  header = "sandbox_features.h"
+  flags = [ "USE_SECCOMP_BPF=$use_seccomp_bpf" ]
+}