Add callback argument to track completion of dump

What I forgot to mention is that HandleScopes are recursive, i.e.:

```c++
Local<Number> v;
{
  HandleScope handle_scope;
  {
    HandleScope handle_scope;
    v = Number::New(42);
  }
  // |v| is no longer valid.
}
```

That means if you don't create a HandleScope here, any handles that are created (including the return value from node::MakeCallback()) leak into the process-wide HandleScope, meaning they stay around forever.  And if there isn't a process-wide HandleScope, V8 will simply abort.