Hide heap methods where possible.

Factory is already a friend class of Heap.
We introduce a TestHeap class in cctest.h to access protected methods.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/247263003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21053 ce2b1a6d-e550-0410-aec6-3dcde31c8c00