cryptohome: Add checksum to critical persistent files.

Checksum status does not interfere with cryptohome operation but it does
report a metric and any mismatches are logged.

BUG=chromium:554709
TEST=manual: logged in and verified vault files have checksums
TEST=manual: verified metrics are being recorded
TEST=unit

Change-Id: Iafda6be43cffc21837b7e9e9441fe8ec15dc4a47
Reviewed-on: https://chromium-review.googlesource.com/312718
Commit-Ready: Darren Krahn <dkrahn@chromium.org>
Tested-by: Darren Krahn <dkrahn@chromium.org>
Reviewed-by: Utkarsh Sanghi <usanghi@chromium.org>
7 files changed