minijail: return change_gid_ from change_gid(), not change_uid_.
minijail: return change_gid_ from change_gid(), not change_uid_.

This change makes platform_MiniJailUidGid pass.

BUG=chromium-os:2110,chromium-os:5327
TEST=platform_MiniJailUidGid

Change-Id: I3ff33b3670ad91668cea6f38e569ba0e24453365
Signed-off-by: Elly Jones <1693f95ea06ee6d7b2276cb599c3f4f33a951d9c@chromium.org>
Reviewed-on: http://gerrit.chromium.org/gerrit/3734
Reviewed-by: Chris Masone <98e3d65c6b0e9e2391eb4c2e8b1e572b77e544ed@chromium.org>

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