avoid getting a profiling sample while not holding the locker,
because we will not get a stack sample in that case.
Review URL: http://codereview.chromium.org/668063

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4024 ce2b1a6d-e550-0410-aec6-3dcde31c8c00