Track Hydrogen statistics on a per-Isolate basis

This is basically the same fix as the one for --trace-hydrogen, but now for
--hydrogen-stats. Removed a few train wrecks on the way.

Review URL: https://codereview.chromium.org/12481015

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13835 ce2b1a6d-e550-0410-aec6-3dcde31c8c00