Allow arbitrary names for weak references in heap snapshots.

LOG=N
BUG=
R=ulan@chromium.org, yurys@chromium.org

Review URL: https://codereview.chromium.org/146843003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18846 ce2b1a6d-e550-0410-aec6-3dcde31c8c00