Add convenience method to print types for debugging.

R=titzer@chromium.org, svenpanne@chromium.org

Review URL: https://codereview.chromium.org/435403002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22908 ce2b1a6d-e550-0410-aec6-3dcde31c8c00