Ensure we don't overwrite transitions in SetPropertyIgnoreAttributes.

BUG=326155
LOG=y
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/134733011

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18754 ce2b1a6d-e550-0410-aec6-3dcde31c8c00