Support premount mode in GetPreconfiguredSharePaths

- Add helper functions to differentiate getting preconfigured share
  paths for both dropdown and premount modes.

Bug: chromium=878502
Test: compiles
Change-Id: I62f5811e2f320a54b783abfdb9b0ffd719abc68a
Reviewed-on: https://chromium-review.googlesource.com/c/1407658
Commit-Queue: jimmy gong <jimmyxgong@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Reviewed-by: Bailey Berro <baileyberro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625418}
3 files changed