Bugfix: AllocationSite objects need to be walkable by the heap snapshot
generator.

BUG=
R=machenbach@chromium.org

Review URL: https://codereview.chromium.org/18584007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15575 ce2b1a6d-e550-0410-aec6-3dcde31c8c00