Use Unique<Object> in HConstant and remove UniqueValueId.

BUG=
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/24350014

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16911 ce2b1a6d-e550-0410-aec6-3dcde31c8c00