SubStringKey::AsHandle is not GC-safe because the string backing store
may move.

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/484703002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23185 ce2b1a6d-e550-0410-aec6-3dcde31c8c00