Clean up PrefetchNextRegion.
Clean up PrefetchNextRegion.

Just to make it a little easier to read.

gfe-relnote: n/a, no functional change in QUIC v99-only method.
PiperOrigin-RevId: 254782688
Change-Id: Icb89655218af852b9da513f6313e91d0a1f973d0

google/quiche,google/quiche,google/quiche,google/quiche