Change cast to String to cast to Symbol in heap-snapshot-generator.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/684893002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24959 ce2b1a6d-e550-0410-aec6-3dcde31c8c00