Use HLoadNamedField to load the string length field instead.

Depends on: https://codereview.chromium.org/21488002

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/21499002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16002 ce2b1a6d-e550-0410-aec6-3dcde31c8c00