Added options --preemption/--no-preemption and --preemption-interval to control preemption in the developer shell when running several threads. Default is still to enable preemption with interval 1ms if threads are created.
Review URL: http://codereview.chromium.org/48001

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1518 ce2b1a6d-e550-0410-aec6-3dcde31c8c00