ChromeOS: Replace PO2TS::GetPrimaryAccountId() with SMB::GetAuthenticatedAccountId.

The functionality is the same, just moving the code to a new place.

BUG=333995

Review URL: https://codereview.chromium.org/135183009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247369 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247369