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