The main fix is for current_state() and external_callback() accessors.
I also applied memory access ordering on current_state_ modification,
mainly to reflect the fact that it is being shared among VM and profiler
sampler threads.

BUG=361

Review URL: http://codereview.chromium.org/2852047

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5035 ce2b1a6d-e550-0410-aec6-3dcde31c8c00