Revert of Linux Sandbox: move init processes to new session id.

Revert of Linux Sandbox: move init processes to new session id. (patchset #1 id:1 of https://codereview.chromium.org/837083002/)

Reason for revert:
Preemptively revert in case it caused a performance regression described in https://crbug.com/447164 even though it seems unlikely.

Original issue's description:
> Linux Sandbox: move init processes to new session id.
>
> The setuid sandbox creates new PID namespaces. Make sure to have
> the new init processes be inside a new session id and process group
> so that children can't signal processes outside of the PID namespace.
>
> BUG=446680
>
> Committed: https://crrev.com/f48cead580e6c82a8f3c4ac2a493249da780ee28
> Cr-Commit-Position: refs/heads/master@{#310394}

TBR=mdempsky@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=446680, 447164
Committed: https://crrev.com/419f5a54506d5b50730e9be432f9363ed1b62f07
Cr-Commit-Position: refs/heads/master@{#311098}

[jln (very slow on Chromium), 2015-01-12 20:14:39]

Created Revert of Linux Sandbox: move init processes to new session id.

[commit-bot: I haz the power, 2015-01-12 20:15:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/847723003/1

[commit-bot: I haz the power, 2015-01-12 20:18:51]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-12 20:20:15]

Patchset 1 (id:??) landed as
https://crrev.com/419f5a54506d5b50730e9be432f9363ed1b62f07
Cr-Commit-Position: refs/heads/master@{#311098}

[mdempsky, 2015-01-12 20:22:16]

lgtm