Move broker_process_ from policies to SandboxLinux itself

Helps untangle the pre-sandbox hooks for subsequent use with
standalone services.

Bug: 708738
Change-Id: Ia4bdcd4852aef80e33374df66e84eb8c5b2f51ec
Reviewed-on: https://chromium-review.googlesource.com/740670
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512261}
5 files changed