- Removed the potential for a NULL pointer access in
  ContextSwitcher::PreemptionReceived.
- Removed a leak of the semaphore in the ContexSwitcher thread, by removing
  the need for this semaphore entirely.
- Added a regression test case which will catch accesses to the ContextSwitcher
  singleton after it has been stopped.

Review URL: http://codereview.chromium.org/14483

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@994 ce2b1a6d-e550-0410-aec6-3dcde31c8c00