Add premount mode for NetworkFileSharesPreconfiguredShares

- The "premount" mode lets preconfigured shares to be preemtively
  mounted upon start up of SMB file sharing.

Bug: chromium=878502
Test: compiles + PolicyPrefTest unittest
Change-Id: I3d53b33bb4687217b537b45d1562c4e4ed1b31b6
Reviewed-on: https://chromium-review.googlesource.com/c/1407655
Reviewed-by: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Commit-Queue: jimmy gong <jimmyxgong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625346}
2 files changed