Fix putting of prototype transitions. The length is also subject to GC, just like entry.

BUG=347536
LOG=n
R=danno@chromium.org

Review URL: https://codereview.chromium.org/183193003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19586 ce2b1a6d-e550-0410-aec6-3dcde31c8c00