libminijail: only clear supplemental groups on user/group change
libminijail: only clear supplemental groups on user/group change

minijail should be runnable by an unprivileged user.  This change allows
that to be true.

BUG=chromium-os:19459
TEST=minijail -S somepolicy /bin/ls
     (need to test transitions still)

Change-Id: Ib540953ae2435414b3d3adbadb68238962f5c0ff
Reviewed-on: http://gerrit.chromium.org/gerrit/7912
Reviewed-by: Elly Jones <1693f95ea06ee6d7b2276cb599c3f4f33a951d9c@chromium.org>
Tested-by: Will Drewry <72416dde44ea762d19e3a7a555e3b59ce98f8eea@chromium.org>

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