Add callback argument to track completion of dump

You need a HandleScope if you don't want to leak handles:

```c++
Isolate* isolate = /* ... */;
HandleScope COMPAT(handle_scope, handle_scope(isolate));
```

You can get the isolate with Isolate::GetCurrent() but maybe it's better to be consistent with the other signal handle and assign the isolate to handle->data in PlatformInit().