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The username and workgroup of ChromAD users will now be passed to
the SmbProvider Mount function in the smbprovider Dameon.

The ParseUserPrincipalName function and corresponding tests are adapted
from AuthPolicy's SambaHelper.

BUG=chromium:757625
TEST=unittests

Change-Id: Ia25396224d9c873161adb2da3741c54cdd1209cf
Reviewed-on: https://chromium-review.googlesource.com/1052914Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558339}