Refactor function.arguments accessor.

This prepares for API-style accessor conversion.

BUG=
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/253453002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20943 ce2b1a6d-e550-0410-aec6-3dcde31c8c00