[heap] API objects with elements aren't unmodified

Bug: v8:6284

Change-Id: I3a52819f4e67a143a0b3b63358fc4dd0df384553
Reviewed-on: https://chromium-review.googlesource.com/483482
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44766}

src/heap/heap.cc

@@ -2969,6 +2969,7 @@ bool Heap::IsUnmodifiedHeapObject(Object** p) {
   Object* maybe_constructor = js_object->map()->GetConstructor();
   if (!maybe_constructor->IsJSFunction()) return false;
   JSFunction* constructor = JSFunction::cast(maybe_constructor);
+  if (js_object->elements()->length() != 0) return false;
 
   return constructor->initial_map() == heap_object->map();
 }