Ensure mutual exclusion on access to GDBJITInterface internal structures to make it work correctly with several isolates.

Review URL: http://codereview.chromium.org/6816053

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7557 ce2b1a6d-e550-0410-aec6-3dcde31c8c00