Switch login_MultipleSessions to use CryptohomeProxy

Instead of shelling out to the cryptohome command line tool,
use CryptohomeProxy to speak to cryptohomed over DBus.

BUG=chromium:344862
TEST=login_MultipleSessions

Change-Id: Ib79cf092b2385b2d922abbc81cf6bd744fe8a0da
Reviewed-on: https://chromium-review.googlesource.com/190088
Tested-by: Chris Masone <cmasone@chromium.org>
Reviewed-by: Will Drewry <wad@chromium.org>
Commit-Queue: Chris Masone <cmasone@chromium.org>
2 files changed