Classes: Add support for toString

BUG=v8:3330
LOG=Y
R=dslomov@chromium.org

Review URL: https://codereview.chromium.org/624013005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24472 ce2b1a6d-e550-0410-aec6-3dcde31c8c00