Assert incumbent global is always some in get_incumbent_global hook

While working on module script, I'd like to use Promise with a custom
callback with type `Box<dyn TaskBox>` which means we didn't use web API
callbacks as native handlers. However, we'll get a panic from
`enqueue_promise_job` and the panic says we have a `null` incumbent
global. The main problem here is, the Promise API is strongly tied to JS
engine and it always assumes there's a meaningful answer for "what
specific global is this promise associated with". So, when I don't use
the Promise for a specific web API, our engine cannot find a proper
incumbent global for us so that we get the `null` incumbent global
panic.

To make us catch this case easier in the future, we should add the
assertion inside `get_incumbent_global` hook so that we can know this
quickly next time.

Ref: https://mozilla.logbot.info/servo/20190807#c16525481

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