minijail: Support writing child pid to file

BUG=chromium:519154
TEST=security_Minijail0 passes

Change-Id: Icedff5d86ef0c3dbf2933e763b0858cb79e5b08f
Reviewed-on: https://chromium-review.googlesource.com/292342
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Commit-Queue: Nicolas Boichat <drinkcat@chromium.org>
Trybot-Ready: Nicolas Boichat <drinkcat@chromium.org>
Tested-by: Yu-hsi Chiang <yuhsi@google.com>
3 files changed