Thread: delete racy API (Release()) and fix racy code (started()).

- Thread::Release() wrote a local variable on the calling thread but read it on
  another thread, with no synchronization.  Happily it has no non-test callers
  so deleting it instead of trying to fix it (see bug for details).
- Thread::started_ similarly was racily being written to; replaced with a
  running_ Event, and hid the accessor except for tests & legacy callers,
  with a note about why it's a bad idea.

webrtc/base patched with:
git diff origin --relative=talk/base | patch -p1 -dwebrtc/base
followed by manual merge of 3 thunks that ran afoul of naming differences
between talk/base and webrtc/base.

BUG=3388
R=andrew@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/14589005

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6236 4adac7df-926f-26a2-2b94-8c16560cd09d