This is the first in a series of CLs that will separate the JS API
from the implementation of WebAssembly by bottlenecking interactions
through the WasmEngine. In the long run, the JS API and much of V8
should rely only on the WasmEngine interface, which will represent
the "public interface" for embedding WebAssembly.

Next: hide compilation-related methods behind WasmEngine.
Bug: v8:7316
Change-Id: I93404f0dc8a201ae99d30b4c1ca34606e3dddbca
Reviewed-on: https://chromium-review.googlesource.com/868590
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50660}