Port r12298 (7b39ef67)

Original commit message:
Now a map points to a transition array which contains the descriptor array. The descriptor array is now immutable. The next step is to share the descriptor array with all back-pointed maps as long as there is a single line of extension. Maps that require a descriptor array but don't need transitions will still need a pseudo-empty transition array to contain the descriptor array.

BUG=
TEST=

Review URL: https://chromiumcodereview.appspot.com/10827335
Patch from Akos Palfi <palfia@homejinni.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12319 ce2b1a6d-e550-0410-aec6-3dcde31c8c00