Add callback argument to track completion of dump

You can find its definition in deps/v8/include/v8.h.  It's a RAII class that tells the garbage collector when it's safe to reclaim values stored in handles.

```c++
Local<Number> v;
{
  HandleScope handle_scope;
  v = Number::New(42);
  v->NumberValue();  // Safe, |v| is rooted in the HandleScope
}  // HandleScope is destructed
v->NumberValue();  // Unsafe, |v| is no longer rooted.
```

(V8 magically turns `v` into an empty handle after the destruction of the HandleScope, actually, but that's not something you should rely on.)