Per request from task scheduler thread.
https://chromium-review.googlesource.com/c/565465 has the prototype; though not 100% done yet, and too complex to land in one piece anyway.
Key difficulty is the need for a replacement mechanism for synchronizing
with disk I/O done by outside clients; also an opportunity since the use of passed-in runner doesn't really work right with Simple.
Comment 1 by morlovich@chromium.org
, Jul 20 2017