Fix data race on Debug::thread_local_.current_debug_scope_

BUG=v8:3614
R=yangguo@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/631223004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24441 ce2b1a6d-e550-0410-aec6-3dcde31c8c00