bug 1039820 - avoid using chromeEventHandler in framescript

the `return` would make sense if there wasn't any other condition after, in order to avoid an additional level of indentation, but that's not the case, so I would suggest something like:
```js
if (!docShell)
  observerSvc.removeObserver(listener, topic);
else if (subject === content.document)
  sendAsyncMessage('sdk/tab/event', { type: EVENTS[topic] });
```
with one line we can avoid to use the brackets – see the coding style guide – but feel free to add if you prefer, the highlight here is on `else if`.