[heap] Assert that there's no recodred slot corresponding to unboxed double field.

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 83859176e410fcca451a8abe127220806b61fc99..1d1cfe7041290be18bae2a719dc2c9094c79a5e3 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -3438,6 +3438,9 @@ void MigrateFastToFast(Handle<JSObject> object, Handle<Map> new_map) {
         // Transition from tagged to untagged slot.
         heap->ClearRecordedSlot(*object,
                                 HeapObject::RawField(*object, index.offset()));
+      } else {
+        DCHECK(!heap->HasRecordedSlot(
+            *object, HeapObject::RawField(*object, index.offset())));
       }
     } else {
       object->RawFastPropertyAtPut(index, value);