Renamed ConvertDescriptorToFieldAndMapTransition to ConvertTransitionToMapTransition, and let it replace the transition in-place rather than copy the transition array.

Review URL: https://chromiumcodereview.appspot.com/10694155

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12071 ce2b1a6d-e550-0410-aec6-3dcde31c8c00