Properly seed the RNG for cctest/test-types.

This was broken since r21879, as the RandomNumberGenerator constructor
does no longer look at FLAG_random_seed implicitly.

TEST=cctest/test-types
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/370973002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22222 ce2b1a6d-e550-0410-aec6-3dcde31c8c00