[wasm] Set correct prototype for WebAssembly.Global

This fixes the first part of a failing spec test, the other WebAssembly
objects will follow in other CLs.

R=jkummerow@chromium.org

Bug: v8:12227
Change-Id: I7b57b0c518671f0614a88f0477b64e2507435aba
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3168272
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76907}

src/wasm/wasm-js.cc

@@ -1362,6 +1362,26 @@ void WebAssemblyGlobal(const v8::FunctionCallbackInfo<v8::Value>& args) {
     return;
   }
 
+  // The infrastructure for `new Foo` calls allocates an object, which is
+  // available here as {args.This()}. We're going to discard this object
+  // and use {global_obj} instead, but it does have the correct prototype,
+  // which we must harvest from it. This makes a difference when the JS
+  // constructor function wasn't {WebAssembly.Global} directly, but some
+  // subclass: {global_obj} has {WebAssembly.Global}'s prototype at this
+  // point, so we must overwrite that with the correct prototype for {Foo}.
+  i::MaybeHandle<i::HeapObject> maybe_prototype =
+      i::JSObject::GetPrototype(i_isolate, Utils::OpenHandle(*args.This()));
+  i::Handle<i::HeapObject> prototype;
+  if (maybe_prototype.ToHandle(&prototype)) {
+    Maybe<bool> result = i::JSObject::SetPrototype(global_obj, prototype,
+                                                   /*from_javascript=*/false,
+                                                   internal::kThrowOnError);
+    if (!result.FromJust()) {
+      DCHECK(i_isolate->has_pending_exception());
+      return;
+    }
+  }
+
   // Convert value to a WebAssembly value, the default value is 0.
   Local<v8::Value> value = Local<Value>::Cast(args[1]);
   switch (type.kind()) {

test/mjsunit/wasm/prototype.js

+// Copyright 2021 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+(function GlobalPrototype() {
+  class _Global extends WebAssembly.Global {}
+  let global = new _Global({value: 'i32', mutable: false}, 0);
+  assertInstanceof(global, _Global);
+  assertInstanceof(global, WebAssembly.Global);
+})();