Implement explicit back pointers in transition tree.
This makes back pointers in the map transition tree explicit by having
accurate back pointers throughout the lifetime of maps instead of
establishing and destroying back pointers before and after each marking
phase. This is a prerequisite for being able to clear map transitions
during incremental marking.
R=vegorov@chromium.org
BUG=v8:1465
Review URL: https://chromiumcodereview.appspot.com/
10381053
git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11528
ce2b1a6d-e550-0410-aec6-
3dcde31c8c00