GitLab

- Adds vectors for both discovery and share callbacks.
- Solves the race condition in which multiple
  dialogs of "Add Smb Share" would use a shared instance of each
  HostLocator.
- If a SmbShareFinder::GatherSharesInNetwork call comes in when one is
  already running, the callbacks are stored and run when the original
  GatherSharesInNetwork call finishes.
- Adds additional unit test to reflect on the changes.

Bug: chromium:892287
Test: end-to-end
Change-Id: I9551532dc08fd9338803786bdd89f0398f4ce7db
Reviewed-on: https://chromium-review.googlesource.com/c/1336435Reviewed-by: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: jimmy gong <jimmyxgong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611409}