The explainer defines the following text:

https://github.com/WICG/background-fetch#reacting-to-abort

> If a background fetch job is aborted, either by the user, or by the developer calling abort() on the background fetch job, the following event is fired in the service worker: [backgroundfetchabort]

This bug is two-folded.
  1) When the developer calls BackgroundFetchManager.abort(), we need to end up calling the event.
  2) When the user aborts the fetch through a user interface, we also need to call the event.

I'll pick up (1), and will then reassign to harkness@ who is working on enabling (2).