Pass the proper number of fields to update remembered set for.

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index 13ffa29c36b7d5eac279f06ad890b395a8696101..a9754ce057837968f482b1920e1e8fc416f972e0 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -2648,7 +2648,7 @@ Object* Heap::CopyJSObject(JSObject* source) {
     // Update write barrier for all fields that lie beyond the header.
     RecordWrites(clone_address,
                  JSObject::kHeaderSize,
-                 object_size - JSObject::kHeaderSize);
+                 (object_size - JSObject::kHeaderSize) / kPointerSize);
   } else {
     clone = new_space_.AllocateRaw(object_size);
     if (clone->IsFailure()) return clone;