[User Timing] Clarification of negative duration values

In working on the implementation of the User Timing spec in Gecko, it was
brought up about the potential of durations to return negative values.
Specifically it was unclear from the spec whether things like this should
be allowed:

```js
performance.mark('A');
performance.mark('B');

performance.measure('delta', 'B', 'A');

assert( performance.getEntriesByName('delta')[0].duration < 0 ); // true
```

Chrome allows this interaction, enabling negative duration values for
measures. Is this intended, and if so, should we clarify the spec that
negative durations are allowed for performance measurements specifically?

Eli Perelman
Mozilla