The tag tells whether the content is ASCII or UC16, or even if the string wasn't flat.

BUG: v8:1633

Review URL: http://codereview.chromium.org/7709024

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8999 ce2b1a6d-e550-0410-aec6-3dcde31c8c00