The rename makes it clear that the atomic counter is an approximation
only. Explanation is added about the update of the counter (increased
when units are added, and reduced to zero if a worker finds no more
units). The comment also sais why it's safe to use relaxed memory
ordering in this case.

R=thibaudm@chromium.org, ahaas@chromium.org

Bug: chromium:1101340
Change-Id: I307d646189bc5732d50c92a94b2a654fa6a7f763
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2410185
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69908}