[login_manager] Fix race condition that caused ownership to never work

There's a race to create the user's NSSDB between chrome and the
session manager.  If the session manager wins, that directory and the
files within are owned by root, which is bad.

TEST=unit, and suite_Smoke (incl login_OwnershipTaken)

Change-Id: I58080f281408c1f8e3d6692f5759148667a7bdff

Review URL: http://codereview.chromium.org/6820024
4 files changed