[wasm] [in-place weak refs] Simplify WeakArrayList usage.

MaybeObjectHandle already has an API for making a weak pointer.

BUG=v8:7308

Change-Id: I9f8390e0dffec850a24391e7f50a455b86b217f5
Reviewed-on: https://chromium-review.googlesource.com/1125922Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54247}

src/wasm/wasm-objects.cc

@@ -1250,9 +1250,7 @@ Handle<WasmInstanceObject> WasmInstanceObject::New(
   Handle<WeakArrayList> weak_instance_list(module_object->weak_instance_list(),
                                            isolate);
   weak_instance_list = WeakArrayList::AddToEnd(
-      weak_instance_list,
-      MaybeObjectHandle(
-          MaybeObject::MakeWeak(MaybeObject::FromObject(*instance)), isolate));
+      weak_instance_list, MaybeObjectHandle::Weak(instance));
   module_object->set_weak_instance_list(*weak_instance_list);
 
   return instance;