The task count was reset after handling the first task, so the wait
loop would end after one iteration. As a result unmapping tasks might
have been left running even after waiting for the tasks to be
completed.

Bug: v8:7412
Change-Id: I869848477946589d0370ab038d135393e816a7c0
Reviewed-on: https://chromium-review.googlesource.com/903166Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51127}