The WasmEngine is shared across the whole process, so there is no need
to store it in every Isolate.
Instead, we can just get it from everywhere on any thread using
{wasm::GetWasmEngine()}, which is a simple read of a global.

R=jkummerow@chromium.org

Bug: v8:11879
Change-Id: I13afb8ca3d116aa14bfaec5a4bbd6d71faa9aa17
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2969825Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75265}