This is only enabled when --experimental-wasm-threads is enabled.

In addition, only shared WebAssembly.Memory may be sent, as specified
here: https://github.com/WebAssembly/design/pull/1074/files#diff-8e85308ab5cc1e83e91ef59233648be2R227

Bug: v8:6895
Change-Id: Id009a7f890d15fa6c98e93f03806f7e7eff30c2a
Reviewed-on: https://chromium-review.googlesource.com/719417
Commit-Queue: Brad Nelson <bradnelson@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48609}