Add an API for enumerating persistent handles

Add an API for enumerating persistent handles

This API lets the embedder enumerate handles that have class ids. WebKit will
use this feature during garbage collection to compute object groups for DOM
nodes. Previously, we kept a list of DOM nodes on the WebKit side, but that
list is redundant with the global handles list in V8.

Review URL: https://codereview.chromium.org/11190011
Patch from Adam Barth <abarth@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12750 ce2b1a6d-e550-0410-aec6-3dcde31c8c00