minijail: add child PID to log messages
minijail: add child PID to log messages

BUG=chromium:248792
TEST=unit tests, manual

Manual test
-----------
- gmerge chromeos-minijail
- reboot
- connect to GoogleGuest
- pkill -STOP wpa_supplicant
- egrep "child process [0-9]+ exited" /var/log/messages

Change-Id: I44923c38f924133ab45700653042c27491d466ba
Reviewed-on: https://gerrit.chromium.org/gerrit/58277
Reviewed-by: Jorge Lucangeli Obes <bc323d654c912a6c99763475eeb77a04da410a4a@chromium.org>
Tested-by: mukesh agrawal <7b3e7a141c6975a7a08771090d3222103f21a0f2@chromium.org>
Commit-Queue: mukesh agrawal <7b3e7a141c6975a7a08771090d3222103f21a0f2@chromium.org>

omegaup/minijail,omegaup/minijail,omegaup/minijail,omegaup/minijail