Fix username validation for device-local accounts policy blobs.

These carry a user name that is determined by the device-local account
device policy configuration, which is an identifier that doesn't
necessarily form a valid email address.

BUG=chromium:333434

Review URL: https://codereview.chromium.org/177073020

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253874 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253874