For variable sized objects this field doesn't really make any sense so
by putting a special value there we can improve SizeFromMap().

Review URL: http://codereview.chromium.org/3127016

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5301 ce2b1a6d-e550-0410-aec6-3dcde31c8c00