Unittest for thread_wrapper.

Added an explanation of the state variables of posix, wrote a simple test.
This might help in fixing the TSAN issue with thread.

BUG=webrtc:300
TEST=

Review URL: https://webrtc-codereview.appspot.com/583008

git-svn-id: http://webrtc.googlecode.com/svn/trunk@2270 4adac7df-926f-26a2-2b94-8c16560cd09d
3 files changed