just indexed string access.

The difference is that String.prototype.charCodeAt accepts any number
as an index, whereas string[] only accepts array indices.

Review URL: http://codereview.chromium.org/2467006

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4790 ce2b1a6d-e550-0410-aec6-3dcde31c8c00