Weak Cells

Introduce an object that holds a weak reference.
Design document: http://goo.gl/9dSvvy.

BUG=
R=erik.corry@gmail.com

Review URL: https://codereview.chromium.org/640303006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24606 ce2b1a6d-e550-0410-aec6-3dcde31c8c00