Added a DependentCode field to AllocationSite. It's not currently used,
this initial CL is just to get the object layout correct.

BUG=
R=hpayer@chromium.org, mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/23567003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16833 ce2b1a6d-e550-0410-aec6-3dcde31c8c00