This is a reland of ac2fb66b.
Crashes were fixed in https://crrev.com/c/1429862.

Original change's description:
> [wasm] Remove finisher task
>
> This removes the finisher task and instead finishes compilation units
> from the background.
> It also changes ownership of the AsyncCompileJob to be shared among all
> tasks that still operate on it. The AsyncCompileJob dies when the last
> reference dies.
>
> R=ahaas@chromium.org
> CC=​​mstarzinger@chromium.org
>
> Bug: v8:7921, v8:8423
> Change-Id: Id09378327dfc146459ef41bc97176a8716756ae4
> Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel
> Reviewed-on: https://chromium-review.googlesource.com/c/1335553
> Reviewed-by: Andreas Haas <ahaas@chromium.org>
> Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#58630}

Bug: v8:7921, v8:8423
Change-Id: I3dcee4e8e56d2a524d302af91b5cb4a7a9ceb8ce
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel
Reviewed-on: https://chromium-review.googlesource.com/c/1400781
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59302}