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This change switches SmbProviderClient to using base::SupportsWeakPtr
and SmbProviderClientImpl to using a helper class that wraps it
instead of a manually implemented WeakPtrFactory.

Bug: chromium:757625
Change-Id: I18f3b04c18ba28bdf8bf9c356b3ebeb98d546cd3
Reviewed-on: https://chromium-review.googlesource.com/956417
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542951}