Use null instead of undefined for deleted elements in code caches.

Use null instead of undefined for deleted elements in code caches.
Update the lookup and update code for code caches to deal with deleted
elements.

Do not clear the code cache for the builtins object.  If there was a
matching element in the code cache, we would have hit the monomorphic
prototype failure case and removed it.
Review URL: http://codereview.chromium.org/3140

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@340 ce2b1a6d-e550-0410-aec6-3dcde31c8c00