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

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

Review URL: https://codereview.chromium.org/687003005

Cr-Commit-Position: refs/heads/master@{#25083}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25083 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Make visualizer robust to graphs with NULL inputs.

R=mstarzinger@chromium.org, jarin@chromium.org
BUG=

Review URL: https://codereview.chromium.org/652263002

Cr-Commit-Position: refs/heads/master@{#25084}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25084 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Fix build because of dead fields.

TBR=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/699673002

Cr-Commit-Position: refs/heads/master@{#25085}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25085 ce2b1a6d-e550-0410-aec6-3dcde31c8c00