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/1335553Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58630}