Show references from weak containers as weak in heap snapshots.

BUG=356590
LOG=Y
R=ulan@chromium.org, yurys@chromium.org

Review URL: https://codereview.chromium.org/213673006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20429 ce2b1a6d-e550-0410-aec6-3dcde31c8c00