Instead of falling back to calling GetObjectProperty we call GetCharAt
directly if the object is a string and the key in a SMI.

Review URL: http://codereview.chromium.org/522015

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3528 ce2b1a6d-e550-0410-aec6-3dcde31c8c00