[heap] Filter out non-heap values when tracing wrappers

We used to rely on the fact that all values kept alive through wrapper
tracing were materialized as heap objects. Smis break this invariant and
need to be filter out.

BUG=chromium:716031

Review-Url: https://codereview.chromium.org/2852463003
Cr-Commit-Position: refs/heads/master@{#44946}

src/heap/heap.cc

@@ -5671,11 +5671,10 @@ void Heap::TracePossibleWrapper(JSObject* js_object) {
 }
 
 void Heap::RegisterExternallyReferencedObject(Object** object) {
+  // The embedder is not aware of whether numbers are materialized as heap
+  // objects are just passed around as Smis.
+  if (!(*object)->IsHeapObject()) return;
   HeapObject* heap_object = HeapObject::cast(*object);
-  if (heap_object == nullptr) {
-    // We might encounter non-empty handles that point to nullptr.
-    return;
-  }
   DCHECK(Contains(heap_object));
   if (FLAG_incremental_marking_wrappers && incremental_marking()->IsMarking()) {
     IncrementalMarking::MarkGrey(this, heap_object);