At the moment, the whole WebAssembly compilation may run in a single
background task. On a low-end device, this can mean that the background
thread is busy for seconds and thereby blocks other tasks, see e.g.
https://crbug.com/914757.

With this CL we re-schedule compilation tasks after every 50ms. These
50ms are an arbitrary number. I don't want to introduce too much
overhead, but since this is in the background we also don't have to
make tasks super short.

Tasks which are going to compile with TurboFan will be posted with
lower priority.

This change requires changes in the CancelableTaskManager. At the
moment it is not possible that a background task posts a new task
which is managed by the same task manager as itself. The problem is
about how to deal with another thread which calls CancelAndWait
concurrently. At the moment, if a new task gets posted after the call
to CancelAndWait, then `CHECK(!canceled_)` in
CancelableTaskManager::Register will fail. If we used a lock to
synchronize the calls to CancelAndWait and Register, then there would
be a deadlock, where the thread which calls CancelAndWait waits for
the task which wants to call Register, but at the same time blocks that
task by holding the lock.

With the change here, posting a task after the call to CancelAndWait
will just immediately cancel the new task. This matches the behavior
you would get if CancelAndWait is called right after calling Register.

Bug: chromium:914757
Change-Id: I6d57aba161db8a915ec0d745658e0c28d25219a8
Reviewed-on: https://chromium-review.googlesource.com/c/1411884
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58898}