fix a flow type error within ScaleTrack
fix a flow type error within ScaleTrack

hammerlab/pileup.js,hammerlab/pileup.js,snaheth/pileup.js,snaheth/pileup.js,hammerlab/pileup.js,snaheth/pileup.js,piotr-gawron/pileup.js,hammerlab/pileup.js,piotr-gawron/pileup.js,piotr-gawron/pileup.js,piotr-gawron/pileup.js,snaheth/pileup.js