This CL enables adding code to the top-most {WasmCodeRefScope} when
either adding code to the {NativeModule} (we have a nice choke point
for that in {PublishCodeLocked}), or when looking up existing code from
a {NativeModule}.
It also enables the DCHECK that for each such operation, there must be
an enclosing {WasmCodeRefScope}.

R=titzer@chromium.org

Bug: v8:8217
Change-Id: Ie93ca0b31b1577bef074923b3d228a9f214a909c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1533861
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60608}