[heap] VerifyPointersVisitor should check that non-HeapObjects are Smis

This brings VerifyPointersVisitor into line with Object::VerifyPointer.

R=hpayer@chromium.org
BUG=v8:5854

Review-Url: https://codereview.chromium.org/2640623002
Cr-Commit-Position: refs/heads/master@{#42471}

src/heap/heap-inl.h

@@ -850,6 +850,8 @@ void VerifyPointersVisitor::VisitPointers(Object** start, Object** end) {
       HeapObject* object = HeapObject::cast(*current);
       CHECK(object->GetIsolate()->heap()->Contains(object));
       CHECK(object->map()->IsMap());
+    } else {
+      CHECK((*current)->IsSmi());
     }
   }
 }