Convert function.length to API-style accessor.

TEST=mjsunit/function-length-accessor
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/257423009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20937 ce2b1a6d-e550-0410-aec6-3dcde31c8c00