See discussion after this CL: https://crrev.com/c/1297960
We want to avoid the link from NativeModule to WasmEngine to enforce
encapsulation. If someone needs access to the WasmEngine, we should
give them a direct pointer.

R=titzer@chromium.org

Bug: v8:8217
Change-Id: I5bb6f4bf9b56c43085786d7092151d51bd0ff3ca
Reviewed-on: https://chromium-review.googlesource.com/c/1304433Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57076}