Load locales files from the CDN when in a web context (bug 1079894)

https://bugzilla.mozilla.org/show_bug.cgi?id=1079894
could you throw in a few newlines so this is more readable?

alternatively, you could do something like this (although I can't really argue that it's more readable :smiling_imp:):

```js
var url = '';
if (media_url) {
    url += '/media/';
}
if (repo) {
  url += repo + '/';
}
url += 'locales/' + locale + '.js';
if (build_id) {
  url += '?b=' + build_id;
}
return url;
```

```js
var url = '';
if (media_url) { url += '/media/'; }
if (repo) { url += repo + '/'; }
url += 'locales/' + locale + '.js';
if (build_id) { url += '?b=' + build_id; }
return url;
```

or you could ignore my suggestions, and just add a few newlines to what you have. I wouldn't blame you heh.