Restore Function()'s expected string representation.

R=rossberg@chromium.org
BUG=v8:2470
TEST=mjsunit/regress/regress-2470

Review URL: https://codereview.chromium.org/12687002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13880 ce2b1a6d-e550-0410-aec6-3dcde31c8c00