Serialization was failing due to unset dominator pointers.

TEST=test-heap-snapshot/AggregatedHeapSnapshotJSONSerialization

Review URL: http://codereview.chromium.org/5314003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5888 ce2b1a6d-e550-0410-aec6-3dcde31c8c00