Introduce phantom weak handles in the API and use them internally for debug info

R=jochen@chromium.org, ulan@chromium.org
BUG=

Review URL: https://codereview.chromium.org/649563006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24899 ce2b1a6d-e550-0410-aec6-3dcde31c8c00