GitLab

- Adds UpdateMountCredentials to SmbProviderClient.
- This function will interact with Dbus to call CrOS's
  SmbProvider::UpdateMountCredentials.
- Updated FakeSmbProviderClient.

Bug: chromium:913691
Test: emerges
Change-Id: I6311bc0d38282dacd659f82348189f96d29526bb
Reviewed-on: https://chromium-review.googlesource.com/c/1388029Reviewed-by: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619852}