Preserve order of fields when doing slow-to-fast object migration.

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/614883003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24594 ce2b1a6d-e550-0410-aec6-3dcde31c8c00