SnapshotCreator: start from existing snapshot if we have one

src/snapshot/deserializer.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_DESERIALIZER_H_
 #define V8_SNAPSHOT_DESERIALIZER_H_
 
 #include "src/heap/heap.h"
 #include "src/objects.h"
 #include "src/snapshot/serializer-common.h"
@@ skipping 16 matching lines @@
 
 // A Deserializer reads a snapshot and reconstructs the Object graph it defines.
 class Deserializer : public SerializerDeserializer {
  public:
   // Create a deserializer from a snapshot byte source.
   template <class Data>
   explicit Deserializer(Data* data, bool deserializing_user_code = false)
       : isolate_(NULL),
         source_(data->Payload()),
         magic_number_(data->GetMagicNumber()),
+        num_extra_references_(data->GetExtraReferences()),
         next_map_index_(0),
         external_reference_table_(NULL),
         deserialized_large_objects_(0),
         deserializing_user_code_(deserializing_user_code),
         next_alignment_(kWordAligned) {
     DecodeReservation(data->Reservations());
   }
 
   ~Deserializer() override;
 
@@ skipping 71 matching lines @@
                                Object** current);
 
   // Cached current isolate.
   Isolate* isolate_;
 
   // Objects from the attached object descriptions in the serialized user code.
   List<Handle<HeapObject> > attached_objects_;
 
   SnapshotByteSource source_;
   uint32_t magic_number_;
+  uint32_t num_extra_references_;
 
   // The address of the next object that will be allocated in each space.
   // Each space has a number of chunks reserved by the GC, with each chunk
   // fitting into a page. Deserialized objects are allocated into the
   // current chunk of the target space by bumping up high water mark.
   Heap::Reservation reservations_[kNumberOfSpaces];
   uint32_t current_chunk_[kNumberOfPreallocatedSpaces];
   Address high_water_[kNumberOfPreallocatedSpaces];
   int next_map_index_;
   List<Address> allocated_maps_;
@@ skipping 10 matching lines @@
 
   AllocationAlignment next_alignment_;
 
   DISALLOW_COPY_AND_ASSIGN(Deserializer);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_SNAPSHOT_DESERIALIZER_H_