The js-to-wasm wrappers are shared across instances, so we cannot
directly call the instance-specific wasm-to-js wrappers. Instead, we
need to call via the import table.

R=titzer@chromium.org

Bug: chromium:843563
Change-Id: Ia882604f6769472fe2eb69176cbed728215ced29
Reviewed-on: https://chromium-review.googlesource.com/1064610Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53254}