The vector of callbacks can only be accessed from main threads.
Otherwise we get flaky data races. Those showed up after removing the
finisher task (https://crrev.com/c/1335553/2).

R=mstarzinger@chromium.org

Bug: v8:7921
Change-Id: I0429ae87427601952723f6e3ad1e02eb0e59a6e1
Reviewed-on: https://chromium-review.googlesource.com/c/1378174
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58244}