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Add the infrastructure for a DownloadDriver recovery flow.  This CL
doesn't actually perform any cleanup or recovery of a DownloadDriver,
but it just returns success.  Implementing the actual recovery code will
be included in a follow up, but this allows us to implement the
DownloadService comprehensive recovery code without blocking on the
implementation of this component in particular.

BUG=736222

Change-Id: Ibbbb8d64b5114593404339d514bafb8b7d984a9b
Reviewed-on: https://chromium-review.googlesource.com/569073
Commit-Queue: Xing Liu <xingliu@chromium.org>
Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486654}