Don't overwrite transition array map while iterating over the transition tree.

Instread of using the transition array map to store the iteration index, we are using the constructor field of the map. Moreover, I removed the unchecked transition array accessors.

BUG=
R=mstarzinger@chromium.org, verwaest@chromium.org

Review URL: https://codereview.chromium.org/223533002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20503 ce2b1a6d-e550-0410-aec6-3dcde31c8c00