Merged r13730 into trunk branch.

Fix overflow in WriteQuoteJsonString and SlowQuoteJsonString

R=ulan@chromium.org
BUG=

Review URL: https://codereview.chromium.org/12310141

git-svn-id: https://v8.googlecode.com/svn/trunk@13737 ce2b1a6d-e550-0410-aec6-3dcde31c8c00