rAc: fix race condition in test

if you don't fake a user name, it might get a username fetch failure at an inopportune moment.

BUG=none
TEST=waterfall stops flaking
R=csharp@google.com

Review URL: https://codereview.chromium.org/56033004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232437 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232437