Based on MR 17 by plata-gl, thank you very much, and thanks to everybody
who answered the survey linked in that MR whether to make this default
or not.
I've left out padding-top since the additional 1px bar towards the top
looks like a bug, I've compared with Fennec on Android and it doesn't
have such a line. Also I've left out the inFullscreen lines from the MR,
not sure what they are for - we can still add them in a follow-up patch
if desired.
