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- Adds a dropdown to select authentication method that is only visible
to chromad users.
- This change has no effect on non-chromad users
- The KerberosAuthentication_ boolean will be passed to
SmbService::Mount in a following CL

Bug: chromium:853803
Change-Id: Icc9ec65a4eb0ae20e90088f2767c7fda17578a0a
Reviewed-on: https://chromium-review.googlesource.com/1191923
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Scott Chen <scottchen@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596257}