Bug 918970 - display existing thread when sending a sms from contact app

old PR : https://github.com/mozilla-b2g/gaia/pull/16216

maybe you can rewrite like this, to make this clearer:
```javascript
if (isIncoming) {
  return Utils.probablyMatches(message.sender, number);
}
```

and then you don't need the `if (isOutgoing)` condition (and so you don't need the bool `isOutgoing` either)