SnapshotCreator: start from existing snapshot if we have one

src/snapshot/snapshot.h

 // Copyright 2006-2008 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_SNAPSHOT_H_
 #define V8_SNAPSHOT_SNAPSHOT_H_
 
 #include "src/snapshot/partial-serializer.h"
 #include "src/snapshot/startup-serializer.h"
 
@@ skipping 21 matching lines @@
   Vector<const byte> Payload() const;
 
   Vector<const byte> RawData() const {
     return Vector<const byte>(data_, size_);
   }
 
  private:
   bool IsSane();
 
   // The data header consists of uint32_t-sized entries:
-  // [0] magic number and external reference count
-  // [1] version hash
-  // [2] number of reservation size entries
-  // [3] payload length
+  // [0] magic number and (internal) external reference count
+  // [1] API-provided external reference count
+  // [2] version hash
+  // [3] number of reservation size entries
+  // [4] payload length
   // ... reservations
   // ... serialized payload
-  static const int kCheckSumOffset = kMagicNumberOffset + kInt32Size;
-  static const int kNumReservationsOffset = kCheckSumOffset + kInt32Size;
+  static const int kNumReservationsOffset = kVersionHashOffset + kInt32Size;
   static const int kPayloadLengthOffset = kNumReservationsOffset + kInt32Size;
   static const int kHeaderSize = kPayloadLengthOffset + kInt32Size;
 };
 
 class Snapshot : public AllStatic {
  public:
   // Initialize the Isolate from the internal snapshot. Returns false if no
   // snapshot could be found.
   static bool Initialize(Isolate* isolate);
   // Create a new context using the internal partial snapshot.
@@ skipping 51 matching lines @@
 };
 
 #ifdef V8_USE_EXTERNAL_STARTUP_DATA
 void SetSnapshotFromFile(StartupData* snapshot_blob);
 #endif
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_SNAPSHOT_SNAPSHOT_H_