Fix and re-enable KioskEnterpriseTest.

I couldn't reproduce the original failure, but the logs look like an
unrelated crash in the login screen code. Before re-enabling, the test
needed some updates after changes to the identity API implementation
in https://codereview.chromium.org/26715002

BUG=chromium:306611

Review URL: https://codereview.chromium.org/103263004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238970 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238970