Move code logging out of the finisher task. Schedule a separate task
for logging, but only if logging is actually enabled.

R=mstarzinger@chromium.org

Bug: v8:7921
Change-Id: Ib2c7db22c87e60e204096df3e8ef5b354802984f
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_isolates_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/c/1308113
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57165}