Convert use_seccomp_bpf to the BUILDFLAG system.

BUG=589674

Review-Url: https://codereview.chromium.org/2523253002
Cr-Original-Commit-Position: refs/heads/master@{#436715}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 7b1273af06ffb033a47bd1837eb16ce13798a74e

config/features.gni

@@ -79,15 +79,6 @@ declare_args() {
 
 # Additional dependent variables -----------------------------------------------
 
-# The seccomp-bpf sandbox is only supported on five architectures
-# currently.
-# Do not disable seccomp_bpf anywhere without talking to
-# security@chromium.org!
-use_seccomp_bpf =
-    (is_linux || is_android) &&
-    (current_cpu == "x86" || current_cpu == "x64" || current_cpu == "arm" ||
-     current_cpu == "arm64" || current_cpu == "mipsel")
-
 # Enable notifications everywhere except iOS.
 enable_notifications = !is_ios