[Runtime] Re-enable seccomp-bpf sandbox on Linux

Chromium for Linux has 2 sandboxes: setuid that creates a chroot like
sandbox and thus requires a setuid helper binary that we don't ship, and the
seccomp-bpf sandbox that was previously disabled for no reason.

I'm enabling the seccomp-bpf sandbox back.
Merge pull request #679 from tmpsantos/sandbox

[Runtime] Re-enable seccomp-bpf sandbox on Linux