This makes the heuristic for computing the number of parallel tasks
in Scavenger consistent with that in Mark-Compactor.

The patch helps mobile devices where even 1 MB new space can take
10ms to scavenge.

Change-Id: I979de5e8485b93808ea079af2756f53d9b720e10
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1685612Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62566}