Behat scroll broken for app tests

A recent modification made in MDL-70727 has broken some of the E2E tests of the mobile app, as you can see in this travis build.

The source of the problem seems to be the usage of a property called _cntDir on a ViewController. I'm not sure what this property is, because it doesn't appear in Ionic's documentation. But there is a documented method called contentRef that can be used for the same purpose.