I'm not quite sure for what use cases this was intended or whether there already are any GeckoView consumers using this API, but given that bug 1371796 mentioned pull-to-refresh as an example, we probably want to expose the user-visible scroll position here, which means the visual viewport nowadays.

Given the usage example of pull-to-refresh in bug 1371796, downstream consumers will probably more interested in the true visible scroll position of the user within the page, i.e. the visual viewport. Listening for *visual* viewport events will also definitively be required to get the saveAndRestoreState GeckoView test properly working once we switch Gecko's session store helper function to use the *visual* viewport scroll position.

Marking as duplicate because the patch now both depends on parts of bug 1498812, but also blocks later parts of that bug.