Drop the previous Semaphore class from platform files.

Add new Semaphore class using the new TimeDelta class for
the WaitFor() operation. Consistently assert correct behaviour
for the different implementations.

Improve test coverage of the Semaphore class.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/23748003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16473 ce2b1a6d-e550-0410-aec6-3dcde31c8c00