Do not visit smis in the root list during GC.

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index c17794f99b394456583b5bf9d9241777f1bc0d63..93d250813451075bd029c8076749117abdf6eb5e 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -5841,6 +5841,9 @@ void Heap::Verify() {
   VerifyPointersVisitor visitor;
   IterateRoots(&visitor, VISIT_ONLY_STRONG);
 
+  VerifySmisVisitor smis_visitor;
+  IterateRootSmis(&smis_visitor);
+
   new_space_.Verify();
 
   old_pointer_space_->Verify(&visitor);
@@ -6138,6 +6141,12 @@ void Heap::IterateWeakRoots(ObjectVisitor* v, VisitMode mode) {
 }
 
 
+void Heap::IterateRootSmis(ObjectVisitor* v) {
+  v->VisitPointers(&roots_[kRootSmisStart], &roots_[kRootListLength]);
+  v->Synchronize(VisitorSynchronization::kRootSmisList);
+}
+
+
 void Heap::IterateStrongRoots(ObjectVisitor* v, VisitMode mode) {
   v->VisitPointers(&roots_[0], &roots_[kStrongRootListLength]);
   v->Synchronize(VisitorSynchronization::kStrongRootList);