Bug 1501665 patches relax the requirement that the apz.allow_zooming pref must be true to allow resolution zooming. This is so documents displayed in an RDM pane can also be zoomed. All of the callsites for APZAllowZooming() probably should convert to calls to the new AllowZoomingForDocument() function. There may be exceptions. Those callsites may need additional changes in order to have access to a relevant Document pointer.

As mentioned in https://phabricator.services.mozilla.com/D19239#492136 - not all the call sites can actually get a Document pointer because they run on different threads and processes than where the document is available. So I think this will need a different approach (setting a flag that gets propagated everywhere) and this bug will likely end up being INVALID. Leaving without a priority set for now so it remains in the untriaged queue and we can revisit it as the patches on bug 1501665 evolve.