Bugfix: dependent code field in AllocationSite was keeping code objects alive even after context death.

BUG=320532
LOG=Y
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/62803008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17856 ce2b1a6d-e550-0410-aec6-3dcde31c8c00