Fix uninitialized field in Debug::ThreadInit().
Fix uninitialized field in Debug::ThreadInit().

Originally reported by Maxim.Mossienko on the isolates branch.

Review URL: http://codereview.chromium.org/3164020

git-svn-id: b158db1e4b4ab85d4c9e510fdef4b1e8c614b15b@5288 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh