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Adds Truncate to SmbProviderClient and FakeSmbProviderClient

CQ-DEPEND=CL:879387

Bug: chromium:757625
Change-Id: Iff2f62efc059a41ad4d15c3cbb18bafac277c690
Reviewed-on: https://chromium-review.googlesource.com/879566
Commit-Queue: Allen Vicencio <allenvic@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533156}