Merged r13235 into 3.15 branch.

Correctly handle negative codes in String.fromCharCode()

BUG=166553

R=svenpanne@chromium.org

Review URL: https://chromiumcodereview.appspot.com/11852010

git-svn-id: https://v8.googlecode.com/svn/branches/3.15@13357 ce2b1a6d-e550-0410-aec6-3dcde31c8c00