Fix race between VisitSmiRoots and SetStackLimit.

R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/198833006

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@20026 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/heap.cc

@@ -6161,6 +6161,8 @@ void Heap::IterateWeakRoots(ObjectVisitor* v, VisitMode mode) {
 
 
 void Heap::IterateSmiRoots(ObjectVisitor* v) {
+  // Acquire execution access since we are going to read stack limit values.
+  ExecutionAccess access(isolate());
   v->VisitPointers(&roots_[kSmiRootsStart], &roots_[kRootListLength]);
   v->Synchronize(VisitorSynchronization::kSmiRootList);
 }