More special handling of the symbol table during garbage collection.

More special handling of the symbol table during garbage collection.

Mark all objects reachable from the symbols except the symbols
themselves as live (and reachable from strong roots).  This ensures
that if the symbol itself remains alive for any reason, and if it was
a sliced string or cons string backed by an external string, then the
external string will be strongly reachable and therefore not get a
weak reference callback.

Review URL: http://codereview.chromium.org/100344

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1846 ce2b1a6d-e550-0410-aec6-3dcde31c8c00