Handlify and convert string.length to new API-style accessor.

BUG=
R=dcarney@chromium.org, yangguo@chromium.org

Review URL: https://codereview.chromium.org/230693003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20652 ce2b1a6d-e550-0410-aec6-3dcde31c8c00