smbfs: Add fuse-smbfs user and group.

BUG=chromium:939236
TEST=None

Change-Id: Iaa8fc5b2cc54c8082a343864a6cc9bfee1c492d6
Reviewed-on: https://chromium-review.googlesource.com/1725751
Tested-by: Anand Mistry <amistry@chromium.org>
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>