Keep the descriptor properly intact while update the field type.

BUG=v8:3458
LOG=y
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/424093002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22671 ce2b1a6d-e550-0410-aec6-3dcde31c8c00