Change String.prototype.concat to be more like similar functions.

This is just cosmetic: We usually avoid using %FunctionSetLength.

R=dslomov@chromium.org

Review URL: https://codereview.chromium.org/400583002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22447 ce2b1a6d-e550-0410-aec6-3dcde31c8c00