Add field nested_sites to AllocationSite. This field is used to maintain
allocation site information for nested array and object literals.
It's not used productively in this CL, merely maintained in a minimal
way. (that comes next :)).

BUG=
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/23463047

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16912 ce2b1a6d-e550-0410-aec6-3dcde31c8c00