As discussed @ https://chromium-review.googlesource.com/c/chromium/src/+/957761#message-4ba6c1bf637f91507544efc89a31e3e4dd407715
and again @ https://chromium-review.googlesource.com/c/chromium/src/+/957761#message-6d0430e640c82f2d5463259fecdc7fabf945b958

Get rid of task runners for WorkerThreads API (use case is always a
one-off task in which case a static call is fine -- just like in
Chromium's base/task_scheduler/post_task.h)

Calling into V8Platform* from any worker thread is safe, what was previously
unsafe was using an Isolate* from worker threads but Isolate* was dropped
from the new worker threads APIs so this is now irrelevant.

Bug: chromium:817421
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Idd2dbc081edfbcb8985eeb45eb64ffb2555fcf7c
Reviewed-on: https://chromium-review.googlesource.com/978443
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52893}