Allow arbitrary names for weak references in heap snapshots.

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R=ulan@chromium.org, yurys@chromium.org

Review URL: https://codereview.chromium.org/145353003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18838 ce2b1a6d-e550-0410-aec6-3dcde31c8c00