Decouple root visitors from object visitors.

src/snapshot/startup-serializer.h

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_SNAPSHOT_STARTUP_SERIALIZER_H_
 #define V8_SNAPSHOT_STARTUP_SERIALIZER_H_
 
 #include <bitset>
 #include "include/v8.h"
 #include "src/snapshot/serializer.h"
@@ skipping 39 matching lines @@
    private:
     DisallowHeapAllocation no_allocation_;
     HeapObjectToIndexHashMap map_;
     int next_index_;
 
     DISALLOW_COPY_AND_ASSIGN(PartialCacheIndexMap);
   };
 
   // The StartupSerializer has to serialize the root array, which is slightly
   // different.
-  void VisitPointers(Object** start, Object** end) override;
+  void VisitRootPointers(Root root, Object** start, Object** end) override;
   void SerializeObject(HeapObject* o, HowToCode how_to_code,
                        WhereToPoint where_to_point, int skip) override;
   void Synchronize(VisitorSynchronization::SyncTag tag) override;
 
   // Some roots should not be serialized, because their actual value depends on
   // absolute addresses and they are reset after deserialization, anyway.
   // In the first pass over the root list, we only serialize immortal immovable
   // roots. In the second pass, we serialize the rest.
   bool RootShouldBeSkipped(int root_index);
 
   bool clear_function_code_;
   bool serializing_builtins_;
   bool serializing_immortal_immovables_roots_;
   std::bitset<Heap::kStrongRootListLength> root_has_been_serialized_;
   PartialCacheIndexMap partial_cache_index_map_;
   List<AccessorInfo*> accessor_infos_;
   DISALLOW_COPY_AND_ASSIGN(StartupSerializer);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_SNAPSHOT_STARTUP_SERIALIZER_H_