Support profiler stack sampling in any situation. After this change, almost all profiler ticks (except GC ones) have a stack sample data associated.

Tested under Linux, OS X, and Windows.

Review URL: http://codereview.chromium.org/50052

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1565 ce2b1a6d-e550-0410-aec6-3dcde31c8c00