Merged r14224 into trunk branch.

Fix JSON.stringify's slow path wrt sliced strings.

BUG=229923
R=mvstanton@chromium.org

Review URL: https://chromiumcodereview.appspot.com/14170004

git-svn-id: https://v8.googlecode.com/svn/trunk@14226 ce2b1a6d-e550-0410-aec6-3dcde31c8c00