Using 'clusterProperties' can fail with 'can't serialize object of unregistered class' if created in an iframe

Nobody has claimed this yet.

The following example will fail if called from a script which has been loaded in an iframe:
```
map.addSource('earthquakes', {
'type': 'geojson',
'data':
'https://docs.mapbox.com/mapbox-gl-js/assets/earthquakes.geojson',
'cluster': true,
'clusterRadius': 80,
'clusterProperties': {
'mag1': ['+', ['case', mag1, 1, 0]],
'mag2': ['+', ['case', mag2, 1, 0]],
'mag3': ['+', ['case', mag3, 1, 0]],
'mag4': ['+', ['case', mag4, 1, 0]],
'mag5': ['+', ['case', mag5, 1, 0]]
}
});
```
The error is thrown from [web_worker_transfer.js:60](https://github.com/mapbox/mapbox-gl-js/blob/v1.12.0/src/util/web_worker_transfer.js#L60) because the serialization code expects the property `_classRegistryKey` to be set on the `clusterProperties` object's prototype.

However, an object created in a different iframe has a completely different prototype, and so the `_classRegistryKey` is absent and the serialization routine fails.

# Workaround

The workaround is to add the following line to the JavaScript evaluated in different iframes that needs to interact with MapBox JS GL:

```
Object._classRegistryKey = 'Object';
```

As far as I know there isn't a reliable way to check for an object literal, so perhaps this could just be documented somewhere.

Start with src/util/web_worker_transfer.js around line 60 and reproduce the clusterProperties failure from an iframe using the example in the issue. Determine whether cross-iframe object serialization should be handled there or documented, then verify the chosen behavior against the reported error and workaround.

Written by the indexing model from the issue text.