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This change adds Start/ContinueReadDirectory methods to
SmbProviderClient. These map 1:1 to corresponding smbprovider daemon
methods.

Bug: chromium:757625
Change-Id: Idb33b81492949b5499fb402686c211948d736f00
Reviewed-on: https://chromium-review.googlesource.com/1228438
Commit-Queue: Zentaro Kavanagh <zentaro@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594467}