Allow using the namespace sandbox in zygote host.
Currently, this is gated behind the enable-namespace-sandbox switch.
Furthermore, the namespace sandbox is only used if seccomp-bpf is
supported.
BUG=
312380
Committed:
https://crrev.com/660e2d468bf22b4c315463173d0651af4fe6143f
Cr-Commit-Position: refs/heads/master@{#315177}