cppgc: Add missing in construction check to ephemeron processing

We need to check whether ephemeron key is fully constructed to sync the
mark bit before checking it.

Bug: chromium:1246730
Change-Id: I3ba69898202c1df94833a0bc7442b2be0e61694e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3143993Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76678}

src/heap/cppgc/marking-state.h

@@ -289,7 +289,9 @@ void MarkingStateBase::ProcessEphemeron(const void* key, const void* value,
   // Filter out already marked keys. The write barrier for WeakMember
   // ensures that any newly set value after this point is kept alive and does
   // not require the callback.
-  if (HeapObjectHeader::FromObject(key).IsMarked<AccessMode::kAtomic>()) {
+  if (!HeapObjectHeader::FromObject(key)
+           .IsInConstruction<AccessMode::kAtomic>() &&
+      HeapObjectHeader::FromObject(key).IsMarked<AccessMode::kAtomic>()) {
     if (value_desc.base_object_payload) {
       MarkAndPush(value_desc.base_object_payload, value_desc);
     } else {