Make all writes to JSMember.val_ atomic and atomically check for
emptiness in Trace.

Bug: chromium:1056170
Change-Id: Ia7034b9318df081aa61c9b6664903dd4f73402a5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2431569Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70309}