Allow self_size to be larger than 2GB in heap snapshots.

LOG=N
R=dslomov@chromium.org, yurys@chromium.org

Review URL: https://codereview.chromium.org/166383002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19445 ce2b1a6d-e550-0410-aec6-3dcde31c8c00