MapCache simplification. It is now a FixedArray that maps number of properties to a WeakCell with a Map.

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/712943002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25253 ce2b1a6d-e550-0410-aec6-3dcde31c8c00