There is no need to require kRingbufferMaxSize samples for estimating the
compaction speed, as the number is already quite stable with a single sample
(which may include timings from compacting multiple pages).

R=hpayer@chromium.org
BUG=chromium:524425
LOG=N

Review URL: https://codereview.chromium.org/1440423002

Cr-Commit-Position: refs/heads/master@{#31993}