See bug 1364714 comment 42. The freetype 2.8 update causes a (small) change in the font metrics returned for our default sans-serif font on Android, and for some reason this appears to be causing a discrepancy on these reftests. It's not immediately clear to me why font metrics should be important here, though, as the tests don't seem to use any text, just a div with a (huge) specified height.
Also for the record, when adding those tests I recall running into a lot of rounding errors and the numbers that I used for the scrollbar position and size of the div were chosen precisely to avoid much of the rounding error. It's quite likely that the freetype update just perturbed enough stuff to make those particular numbers now trigger a different rounding error so the scrollbar position is off by one. We can probably just adjust the numbers a little to bring the scrollbar position back in alignment.