Let the caller addref it if needed.

I wrote this because I wanted to make some code dealing with it
thread-safe, but I ended up writing a less sketchy solution. However I
still think this is worth it.

It seems this only returns an already_AddRefed because before it used to
be an XPCOM-ish thing where the widget was returned as an out-param.

For now it doesn't change behavior but there are some callers that would
benefit from having less addref/release calls if they only need to read
simple stuff from the widget.

Differential Revision: https://phabricator.services.mozilla.com/D168141