base/thread_unittest: wrap test was setting current thread to NULL.

This broke unittests following ThreadTest.Wrap

BUG=3836
R=pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/28689004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7413 4adac7df-926f-26a2-2b94-8c16560cd09d