Add Contains(), at(), and a constructor with raw addresses to UniqueSet<T> and Unique<T>.

BUG=
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/23872027

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16716 ce2b1a6d-e550-0410-aec6-3dcde31c8c00