Convert use_seccomp_bpf to the BUILDFLAG system.

components/nacl/loader/sandbox_linux/BUILD.gn

Index: components/nacl/loader/sandbox_linux/BUILD.gn
diff --git a/components/nacl/loader/sandbox_linux/BUILD.gn b/components/nacl/loader/sandbox_linux/BUILD.gn
index ce1d40447ff3048441f65bab14ad4985a89df19e..8f61844d0bcf0bb46c5d4371cd6ad4df88218c61 100644
--- a/components/nacl/loader/sandbox_linux/BUILD.gn
+++ b/components/nacl/loader/sandbox_linux/BUILD.gn
@@ -24,14 +24,11 @@ source_set("sandbox_linux") {
     "//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" ]
-  }
 }