Bug 1062805 - Used the shared JSON file load helper in the ringtones app R=Goutam

Please indent this like so:

```
return LazyLoader.getJSON(BASE_URLS[toneType] + 'list.json')
                 .then(function(json) {
  toneDefsCache[toneType] = json;
  return toneDefsCache[toneType];
}, function(error) {
  console.error(error);
  return error;
});
```