R=titzer@chromium.org
TEST=cctest/test-wasm-shared-engine/SharedEngineRunThreadedExecution

Change-Id: I01fbb13408020145c9d6bd6bbb66ca5136922bd2
Reviewed-on: https://chromium-review.googlesource.com/1151629Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54733}