Add NetworkFileSharesAllowed pref and policy

This change adds a NetworkFileSharesAllowed pref and policy to store
whether or not a user is allowed to use the Network File Shares
feature.

Bug: chromium:757625
Change-Id: Icaf2015a841c31328b8cb36d26c73b2fa09fbcb6
Reviewed-on: https://chromium-review.googlesource.com/1141301
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578771}
10 files changed