This is done now while embedders have yet to adapt to the new API before
it becomes hard to migrate.

Also renamed variable/methods to use "worker threads" rather than
"background" nomenclature.

Extracted from https://chromium-review.googlesource.com/c/v8/v8/+/978443/7
while resolving the more contentious bits around using task runners.

TBR=rmcilroy@chromium.org

Bug: chromium:817421
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Ie3ddf15a708e829c0f718d89bebf3e96d1990c16
Reviewed-on: https://chromium-review.googlesource.com/980953
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52231}