psnr: tweak sampling rate to make a new measurement every second more likely

When getStats is polled at exactly 1hz the current frame sampling may
occasionally yield no increase in measurements. Avoid this by doing a
simple linear extrapolation using the previous and current frame and
comparing that extrapolation to the 90k frequency.

Bug: webrtc:375021
Change-Id: I252a2b3fef87cbd728e40f83e7efe18f05aafc37
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/406143
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Philipp Hancke <phancke@meta.com>
Cr-Commit-Position: refs/heads/main@{#45458}