Spinning up a new background task is expensive, and many times an existing
task will finish it's work before a new task starts work on a job, so enable
the existing tasks to do more than one background job.

BUG=v8:5203

Change-Id: Ibbef317c8bb3921c36a096fed88d244716be9c42
Reviewed-on: https://chromium-review.googlesource.com/441706
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43593}