Implement explicit back pointers in transition tree.

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