Implement policy::GetOSUsername for POSIX platforms.

Bug: 847265
Change-Id: I332cd4d5bd6996019e132abbd313e765f4e080d3
Reviewed-on: https://chromium-review.googlesource.com/1082662
Reviewed-by: Owen Min <[email protected]>
Reviewed-by: Julian Pastarmov <[email protected]>
Commit-Queue: Georges Khalil <[email protected]>
Cr-Commit-Position: refs/heads/master@{#567791}
2 files changed