Add Start/ContinueReadDirectory methods to SmbFileSystem

- Implements Start/ContinueReadDirectory methods to SmbFileSystem

Bug: chromium:757625
Change-Id: Ib692fb34b488c951c518d265af19cb147ee40c9e
Reviewed-on: https://chromium-review.googlesource.com/1231816
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595265}
2 files changed