If a map has descendents in the map transition tree that are alive,
it is kept.  Only maps such that they and all their descendants
have no live objects are collected.  This happens in mark-sweep and
mark-compact garbage collections.
Review URL: http://codereview.chromium.org/8099

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@601 ce2b1a6d-e550-0410-aec6-3dcde31c8c00