There's no way to resize a window on Android, but we most likely need to reset the zoom level to 100% to make sure the screenshots are meaningful, comparable and stable. However, it seems like WebDriver specs has no mentioning of zoom level.

I'm looking at document.body.style.zoom, which is unfortunately a Blink/WebKit-only API but perhaps can be justified to use here.

Also related to #7598 #12725

Zoom could and should be easily added to WebDriver.

Note that running reftests on mobile has some interesting challenges. For example you need to ensure that the device pixel ratio is set to 1, even though phones usually have a very different value. Getting the viewport to be the expected size is also a problem. Gecko basically has some special settings that afiak are only used for reftests that allow creating a document with the correct size and getting a screenshot of the full document independent of the actual device size.

@lukebjerring set to backlog priority, but maybe should be higher if it's blocking Android runs?

We can easily bypass reftests on Android for now. The question is how much value we can get out of reftests on Android.