cryptohome: Create fresh destination directory for stateful recovery.

Contents of pre-existing directories can't be trusted because the
stateful file system is susceptible to manipulation by attackers
before boot.

BUG=chromium:766276
TEST=new unit test

Change-Id: Ie5b39c006b2d080ae7e981e7000fca4ebcebb14e
Reviewed-on: https://chromium-review.googlesource.com/674697
Commit-Ready: Mattias Nissler <mnissler@chromium.org>
Tested-by: Mattias Nissler <mnissler@chromium.org>
Reviewed-by: Mattias Nissler <mnissler@chromium.org>
(cherry picked from commit 496f2827e7513f0ab5847e0ec3b8931c1962a3b0)
Reviewed-on: https://chromium-review.googlesource.com/678614
2 files changed