Do not visit smis in the root list during GC.

src/heap-inl.h

Index: src/heap-inl.h
diff --git a/src/heap-inl.h b/src/heap-inl.h
index 35bad4af39ec94937720004f4e760332c2a0e36b..a45e3ab9d9ce1630f6b2a2a6da719fd1b42beb45 100644
--- a/src/heap-inl.h
+++ b/src/heap-inl.h
@@ -820,6 +820,13 @@ void VerifyPointersVisitor::VisitPointers(Object** start, Object** end) {
 }
 
 
+void VerifySmisVisitor::VisitPointers(Object** start, Object** end) {
+  for (Object** current = start; current < end; current++) {
+     CHECK((*current)->IsSmi());
+  }
+}
+
+
 double GCTracer::SizeOfHeapObjects() {
   return (static_cast<double>(heap_->SizeOfObjects())) / MB;
 }