[wasm] Use WeakFixedArray for list of instances sharing a WasmMemoryObject.

test/mjsunit/wasm/wasm-module-builder.js

Index: test/mjsunit/wasm/wasm-module-builder.js
diff --git a/test/mjsunit/wasm/wasm-module-builder.js b/test/mjsunit/wasm/wasm-module-builder.js
index d4b1d69f5cec94b394b644aedd03b1f25fccf251..4bfe73d5e650e5a3254241a0214b19b272a03a73 100644
--- a/test/mjsunit/wasm/wasm-module-builder.js
+++ b/test/mjsunit/wasm/wasm-module-builder.js
@@ -623,4 +623,8 @@ class WasmModuleBuilder {
     let instance = new WebAssembly.Instance(module, ffi);
     return instance;
   }
+
+  toModule(debug = false) {
+    return new WebAssembly.Module(this.toBuffer());
+  }
 }