Bug 1007774 - Part 2: fix undefined/null error found with hamachi.

Ah, I see what's going on now. In that case, you don't need to explicitly set `undefined`, you can do:

```js
var perfGoal = mozPerfGoal && mozPerfGoal[perfGoalKey];
```

That will set `perfGoal` to `undefined` when `mozPerfGoal` is `undefined`, and will set it to the correct value if it's not `undefined`.