Some jobs might need to be aborted, e.g., if a function is a default parameter in an
arrow function it will be re-scoped and won't have a SFI to register. Adds support to
abort jobs without having to block if the job is currently running on the background
thread.

BUG=v8:8041

Change-Id: I9149740401cbaaa31c21be9d79d4e3f5c450bfcf
Reviewed-on: https://chromium-review.googlesource.com/c/1278497
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56637}