Use SupportsWeakPtr<SmbFileSystem>

This change updates SmbFileSystem to using SupportsWeakPtr instead of
manually implementing a WeakPtrFactory.

Bug: chromium:757625
Change-Id: I00cf23d863a5f094185217bc5d18bda49d4405db
Reviewed-on: https://chromium-review.googlesource.com/957205
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542917}
2 files changed