Make it possible to use HeapType inside objects-inl.h.

R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/220193007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20390 ce2b1a6d-e550-0410-aec6-3dcde31c8c00