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This change adds a SmbFileSystem::GetWeakSmbProviderClient() method for
getting a weak_ptr to the SmbProviderClient singleton. This weak_ptr will
be used for posting tasks in the form of base::OnceClosure to the SmbTaskQueue.

Bug: chromium:757625
Change-Id: I4e10bfd3b10a243cc65303a15e8f2e1249d1560b
Reviewed-on: https://chromium-review.googlesource.com/957307
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542952}