Specialize ScavengingVisitor for the case when all logging and profiling is disabled.

src/objects-visiting.h

Index: src/objects-visiting.h
diff --git a/src/objects-visiting.h b/src/objects-visiting.h
index 42f90608c35032452fc83b909ef13cd7d9000f61..da955da614f42cd7148f92794afa7b9f9339408e 100644
--- a/src/objects-visiting.h
+++ b/src/objects-visiting.h
@@ -141,13 +141,22 @@ class StaticVisitorBase : public AllStatic {
 template<typename Callback>
 class VisitorDispatchTable {
  public:
+  void CopyFrom(VisitorDispatchTable* other) {
+    // We are not using memcpy to guarantee that during update
+    // every element of callbacks_ array will remain correct
+    // pointer (memcpy might be implemented as a byte copying loop).
+    for (int i = 0; i < StaticVisitorBase::kVisitorIdCount; i++) {
+      NoBarrier_Store(&callbacks_[i], other->callbacks_[i]);
+    }
+  }
+
   inline Callback GetVisitor(Map* map) {
-    return callbacks_[map->visitor_id()];
+    return reinterpret_cast<Callback>(callbacks_[map->visitor_id()]);
   }
 
   void Register(StaticVisitorBase::VisitorId id, Callback callback) {
     ASSERT(id < StaticVisitorBase::kVisitorIdCount);  // id is unsigned.
-    callbacks_[id] = callback;
+    callbacks_[id] = reinterpret_cast<AtomicWord>(callback);
   }
 
   template<typename Visitor,
@@ -179,7 +188,7 @@ class VisitorDispatchTable {
   }
 
  private:
-  Callback callbacks_[StaticVisitorBase::kVisitorIdCount];
+  AtomicWord callbacks_[StaticVisitorBase::kVisitorIdCount];
 };