Instead of reporting kB/s as documented, we were computing bytes/us,
i.e. MB/s, hence mostly reporting 0.
This CL fixes this and renames the variables to make the units more
obvious.

R=titzer@chromium.org

Bug: v8:8975
Change-Id: I0177a8631cc600ab1369657318944f9a215486ce
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1543351Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60509}