In chromium, the platform might delete the task before executing it
and before fully deregistering the Isolate.
In that case we need to deregister it from the WasmEngine to avoid a
data race or use-after-free.

R=mstarzinger@chromium.org
CC=​​herhut@chromium.org

Bug: v8:8783, chromium:928458
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel
Change-Id: Ie94e037f07fbe220505a5d8314b413f24c0990e1
Reviewed-on: https://chromium-review.googlesource.com/c/1454598
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59372}