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This change switches SmbFileSystem to routing all calls to
SmbProviderClient through an SmbTaskQueue.

Test: verified manually on eve
Bug: chromium:757625
Change-Id: Id3466a5cbd0858070bf7170f1358d88bd8d32334
Reviewed-on: https://chromium-review.googlesource.com/957477
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543842}