- Make sure stack guards are properly setup even when preemption is active.
- Fix missing v8::Locker and v8::Unlocker use in d8.cc.
- Threads forked in d8 do get their own context setup.

Review URL: http://codereview.chromium.org/18581

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1161 ce2b1a6d-e550-0410-aec6-3dcde31c8c00