Crankshaft doesn't depend on the sampler thread any more. No
need to start the that thread for Crankshaft.

BUG=v8:2609

Review URL: https://codereview.chromium.org/13526003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14166 ce2b1a6d-e550-0410-aec6-3dcde31c8c00