Is `!pbService` really necessary ? Isn't check for `utils.PrivateBrowsingUtils.isWindowPrivate` enough ?

I think it would be a lot more easier if you did something like follows (assuming I don't miss anything).

```js
function False() { return false };
let isWindowPrivate = (utils.PrivateBrowsingUtils && utils.PrivateBrowsingUtils.isWindowPrivate) ||
                                    False;
```