This zaps the contents of stale descriptor arrays and prototype
transition arrays before overwriting references to them. It should help
to discover accidental sharing early and is needed for the heap verifier
when map collection with incremental marking lands.

R=ulan@chromium.org
BUG=v8:1465

Review URL: https://chromiumcodereview.appspot.com/10383186

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11569 ce2b1a6d-e550-0410-aec6-3dcde31c8c00