Fix building with clang

src/serialize.h

Index: src/serialize.h
diff --git a/src/serialize.h b/src/serialize.h
index 839bfc5618cc33c04b96a8d900c9e97c5b99291a..72eed5ad2f52eb9f15c3b08a41df539b9c2e78d6 100644
--- a/src/serialize.h
+++ b/src/serialize.h
@@ -341,10 +341,6 @@ class Deserializer: public SerializerDeserializer {
   // Deserialize a single object and the objects reachable from it.
   void DeserializePartial(Object** root);
 
-#ifdef DEBUG
-  virtual void Synchronize(const char* tag);
-#endif
-
  private:
   virtual void VisitPointers(Object** start, Object** end);
 
@@ -485,9 +481,6 @@ class Serializer : public SerializerDeserializer {
   SerializationAddressMapper* address_mapper() { return &address_mapper_; }
   void PutRoot(
       int index, HeapObject* object, HowToCode how, WhereToPoint where);
-#ifdef DEBUG
-  virtual void Synchronize(const char* tag);
-#endif
 
  protected:
   static const int kInvalidRootIndex = -1;