Summary:
We're seeing ` inputRange must be monotonically increasing -1,0,0,-33,-32 ` which happens when we
have zero height headers, wherever those come from...maybe rendering null?
The math was also off and didn't handle variable height headers correctly, and it was confusing
because it was `setNextHeaderY` with the header y _minus it's height_, which only works
if the prev height was also the same height.
Reviewed By: furdei
Differential Revision: D4649404
fbshipit-source-id: c2c2d438fa0d0b979c2cbdfa5752eaf86c14768b
94a333a2ea