Auto merge of #20265 - Xanewok:fix-js-objects-in-unions, r=jdm

Fix JS object conversion in unions

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Requires safe `Heap::boxed` constructor from https://github.com/servo/rust-mozjs/pull/395 (more info on it is in the PR).

Since unions currently assume that their respective members root themselves and can be stored on heap, I modified the union member object conversion branch to convert to a `RootedTraceableBox<Heap<*mut JSObject>>` (which is the currently generated type for objects in said unions).

I did it only for Unions and not dictionaries, since some dictionaries had bare `*mut JSObject` members - is this a mistake and something that needs further fixing?

Does this need a test, e.g. passing a union with object to a function that returns said object, and comparing the results for equality?

r? @jdm

Generated code with this patch (for `StringOrObject`):
```rust
impl FromJSValConvertible for StringOrObject {
    type Config = ();
    unsafe fn from_jsval(cx: *mut JSContext,
                         value: HandleValue,
                         _option: ())
                         -> Result<ConversionResult<StringOrObject>, ()> {
        if value.get().is_object() {
            match StringOrObject::TryConvertToObject(cx, value) {
                Err(_) => return Err(()),
                Ok(Some(value)) => return Ok(ConversionResult::Success(StringOrObject::Object(value))),
                Ok(None) => (),
            }

        }
        // (...)
    }
}

impl StringOrObject {
    // (...)
    unsafe fn TryConvertToObject(cx: *mut JSContext, value: HandleValue) -> Result<Option<RootedTraceableBox<Heap<*mut JSObject>>>, ()> {
        Ok(Some(RootedTraceableBox::from_box(Heap::boxed(value.get().to_object()))))
    }
}
```

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- [ ] These changes fix #17011 (github issue number if applicable).

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