It seems that the mix of atomic and non-atomic updates to the same
memory location is not working correctly. One fix is changing all memory
updates to be atomic. Another fix is removing the non-atomic access that
happens while the workers are already running (using atomic accesses).
This CL implements the latter.

R=ahaas@chromium.org

Bug: v8:10647, v8:10650
Change-Id: I84b4f3f442b6be3c4ea6e51962a523f443f5e43b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2273133Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68603}