on_update callback breaks in browser when mode is specified

[sc1f]

When calling View.on_update with the optional mode parameter, an Uncaught DOMException occurs:

Uncaught DOMException: Failed to execute 'postMessage' on 'Worker': function() {
            console.log("updated");
        } could not be cloned.

This issue does not occur internally within the test suite, but occurs whenever we externally call on_update within, for example, a browser context. This is due to something within the Worker API not consuming the callback correctly and failing to cast to a serializable object - I will investigate this further but making a note of it here.