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