image-burner: adding user image-burner to group disk and chronos-access.

This grants user image-burner write access to mounted disk and read
access to files in directory Downloads.

BUG=chromium:766130
TEST=Build image with updated package
chromeos-base/chromeos-imageburner, verify that Recovery Util can burn
CrOS images to usb drives with sandboxed image-burner daemon.

Cq-Depend: chromium:2872959
Change-Id: If83bc9fb6ffbf32ea38885d53f9711c07024fe0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/eclass-overlay/+/2869385
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Reviewed-by: Nicole Anderson-Au <nvaa@google.com>
Reviewed-by: Betul Soysal <betuls@google.com>
Tested-by: Zi Lin <lziest@chromium.org>
Commit-Queue: Zi Lin <lziest@chromium.org>
diff --git a/profiles/base/accounts/group/chronos-access b/profiles/base/accounts/group/chronos-access
index 3a19b96..e3cea79 100644
--- a/profiles/base/accounts/group/chronos-access
+++ b/profiles/base/accounts/group/chronos-access
@@ -4,4 +4,4 @@
 # mostly system daemons running as a non-chronos user, group permissions
 # to access files/directories owned by chronos.
 # This includes all users accessing opencryptoki database files.
-users:root,ipsec,chronos,cros-disks,imageloaderd,crash,dlp
+users:root,ipsec,chronos,cros-disks,imageloaderd,crash,dlp,image-burner
diff --git a/profiles/base/accounts/group/disk b/profiles/base/accounts/group/disk
index f425190..9ce306f 100644
--- a/profiles/base/accounts/group/disk
+++ b/profiles/base/accounts/group/disk
@@ -1,3 +1,3 @@
 group:disk
 gid:6
-users:root,adm,cros-disks,cros_healthd
+users:root,adm,cros-disks,cros_healthd,image-burner