The num_tasks computation has long been based on NumberOfAvailableBackgroundThreads()

We used to have one background worker per core, stealing cycles from
the main thread. I fixed that @ crrev.com/534414. But now this
computation is wrong and generates one less task than it should (one
per worker but the main thread takes task #0 in practice).

Other usage of NumberOfAvailableBackgroundThreads() in V8 seem correct
already so this is the only tweak required.

R=mlippautz@chromium.org

Bug: chromium:808028
Change-Id: I784ed9b764017f146931547d30be4a3b180b5a2c
Reviewed-on: https://chromium-review.googlesource.com/904662
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51121}