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 <ellyjones@chromium.org>
Tested-by: Will Drewry <wad@chromium.org>
1 file changed