ThreadLocalTop used to be static and was zero initialized by the
linker. With isolates we have to give it a constructor.

BUG=http://crbug.com/79393

Review URL: http://codereview.chromium.org/6862005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7635 ce2b1a6d-e550-0410-aec6-3dcde31c8c00