If a web worker terminates while a wasm finisher task is scheduled, we
try to cancel that task even though the platform already deleted it.
This results in UBSan failures, ASan failures or crashes.

This CL fixes this by deregistering the foreground task when it is
deleted before being executed.
A layout test for this will be added to chromium in
https://crrev.com/c/1209602.

R=ahaas@chromium.org

Bug: chromium:875579
Change-Id: Icae43a9dcc6dc16c872851961894bf8bc0872de8
Reviewed-on: https://chromium-review.googlesource.com/1209344
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55689}