We were not disposing the semaphores and the thread used in SourceGroup.

R=mnaganov@chromium.org
Signed-off-by: Thiago Farina <tfarina@chromium.org>

Review URL: http://codereview.chromium.org/7754007/

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9198 ce2b1a6d-e550-0410-aec6-3dcde31c8c00