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For some browser tests, it is useful to create an in-memory instance of
Account Manager.
Allow this by checking the provided cryptohome root directory during
initialization. If it is an empty path, then Account Manager should not
perform any disk I/O.

Bug: 1068240
Change-Id: I1e0278bf65631e01f246ea85074af4e2ca5e4807
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2254780Reviewed-by: Anastasiia N <anastasiian@chromium.org>
Commit-Queue: Kush Sinha <sinhak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781331}