Store API callback entry address prior to entering a callback.

Store API callback entry address prior to entering a callback.

Callback entry address is stored in VMState and is later retrieved by
profiler stack sampler. This makes possible relating API entry to JS
stack, and this is simpler than trying to unwind native stack.

Review URL: http://codereview.chromium.org/437004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3344 ce2b1a6d-e550-0410-aec6-3dcde31c8c00