The predictable platform can make tasks deadlock if the spawning task
is holding a lock that the spawn task also wants to take. This is
because the spawned task is just executed immediately within the
"context" of the spawning task.

The wasm async compile tests deadlock because the
{BackgroundCompileTask}s hold the shared {BackgroundCompileToken}
(reader lock) while spawning new tasks via {OnBackgroundTaskStopped} ->
{RestartBackgroundTasks}. The new tasks might want to cancel
compilation via {BackgroundCompileToken::Cancel}, which takes the
writer lock and hence deadlocks.
This can not happen on any other platform, since tasks are not nested
that way.

R=ahaas@chromium.org

Bug: v8:9760
No-Try: true
Change-Id: I9fc34d5de386aa5c6fdd64a1570fddcff872ec95
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1816502Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63904}