Port r12034 (f17b84c0)

Original commit message:
Bitfield3 now has its own field, while the backpointer shares the field with the descriptor array; which will become the transition array.

BUG=
TEST=

Review URL: https://chromiumcodereview.appspot.com/10692192
Patch from Akos Palfi <palfia@homejinni.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12075 ce2b1a6d-e550-0410-aec6-3dcde31c8c00