Add policy for preconfigured SMB shares

- Adds NetworkFileSharesPreconfiguredShares policy, which allows shares to
be preconfigured by a user. Initially the only supported fields are share_url
and mode. The only supported mode is drop_down, which adds share_url to the
discovery drop down.
- In the future, additional modes will be implemented such as a mode that
automatically mounts the shares. This will also require adding credential
fields to the object.

Bug: chromium:878502
Change-Id: I7494e5f9680f313680add379ac3833610d1213a9
Reviewed-on: https://chromium-review.googlesource.com/c/1265903
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Bartosz Fabianowski <bartfab@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#598926}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 916bede878d540535d4e0651be1cc8a6d530ac21
1 file changed