Make sure that the workers do not start running before the main thread
told them so by setting the memory to the first element in the sequence.
Otherwise it can happen that the main thread resets the memory after the
workers already started doing their updates, which results in a hang
(see linked bug).

R=marja@chromium.org

Bug: v8:10625
Change-Id: I959018279e0049900d44457b72146bc37a12bcb4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2252191
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68429}