This allows more efficient implementations of string keyed access,
String.prototype.chatAt, and String.fromCharCode.

Review URL: http://codereview.chromium.org/6682025

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7160 ce2b1a6d-e550-0410-aec6-3dcde31c8c00