Revert of Start all children in their own PID namespace. (patchset #14 id:360001 of https://codereview.chromium.org/868233011/)

Reason for revert:
Appears to cause system lockups on Chrome OS. See crbug.com/471878

Original issue's description:
> Start all children in their own PID namespace.
>
> BUG=460972
>
> Committed: https://crrev.com/4557699fbb30704f76c68b906d2656d2e322572c
> Cr-Commit-Position: refs/heads/master@{#322660}

TBR=jln@chromium.org,mdempsky@chromium.org,rickyz@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=460972

Review URL: https://codereview.chromium.org/1041163003

Cr-Commit-Position: refs/heads/master@{#322891}