Merged r13853 into 3.17 branch.

Insert missing type cast in JSON.stringify.

BUG=v8:2570
R=danno@chromium.org

Review URL: https://codereview.chromium.org/14840007

git-svn-id: https://v8.googlecode.com/svn/branches/3.17@14514 ce2b1a6d-e550-0410-aec6-3dcde31c8c00