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Add the way for the DownloadService Model to attempt a hard recovery.
This will be used along with other component CLs to allow the
DownloadService to attempt to reset itself.

The recovery method for the Model happens via the following steps:
1. Clears the Model of all Entries.
2. Has the Store destroy itself.
3. The Store attempts to re-initialize.
4. Return the result back to the Model::Client.

BUG=736222

Change-Id: I4e0a256d0b241ef627d457d9c2276691d61da8ce
Reviewed-on: https://chromium-review.googlesource.com/568712
Commit-Queue: David Trainor <dtrainor@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487160}