{ThreadInWasmScope} in only used in a single location, which is in a
test. It does not look like many more such places will show up.

R=titzer@chromium.org,

Bug: v8:8015
Change-Id: I89f6f7e2bd13e2882e65f7657d73ba59a6c71757
Reviewed-on: https://chromium-review.googlesource.com/1193446
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55486}