detecting the ASCII case.  Also rewrite Utf8Length to work on an
unflattened string.  Bug: http://code.google.com/p/v8/issues/detail?id=1665
Review URL: http://codereview.chromium.org/8304021

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9661 ce2b1a6d-e550-0410-aec6-3dcde31c8c00