A benign datarace can occur between the array buffer tracker and
using an arraybuffer as an asm.js memory. The former reads the
{is_shared} bit, which should never change, and the latter writes
the {is_asmjs_memory} bit, but no other bits. Since these bits are
packed into a single word, TSAN reports a race.

R=ulan@chromium.org
BUG=v8:9531

Change-Id: Icceff211368e13794b6678b5fd7748fb5b3235bf
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1714647
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62866}