Abstract string building in JSON-stringifier into IncrementalStringBuilder.

R=ishell@chromium.org

Review URL: https://codereview.chromium.org/713223002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25276 ce2b1a6d-e550-0410-aec6-3dcde31c8c00