buffet: Start buffet under buffet user. BUG=chromium:390697 TEST=manual Change-Id: Ib2e96d361f47da01720cc68522bc1e0209a5d18f Reviewed-on: https://chromium-review.googlesource.com/230850 Tested-by: Anton Muhin <antonm@chromium.org> Reviewed-by: Christopher Wiley <wiley@chromium.org> Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org> Commit-Queue: Anton Muhin <antonm@chromium.org>