SnapshotCreator: start from existing snapshot if we have one

src/snapshot/serializer-common.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_SERIALIZER_COMMON_H_
 #define V8_SNAPSHOT_SERIALIZER_COMMON_H_
 
 #include "src/address-map.h"
 #include "src/external-reference-table.h"
 #include "src/globals.h"
@@ skipping 227 matching lines @@
 
   SerializedData(byte* data, int size)
       : data_(data), size_(size), owns_data_(false) {}
   SerializedData() : data_(NULL), size_(0), owns_data_(false) {}
 
   ~SerializedData() {
     if (owns_data_) DeleteArray<byte>(data_);
   }
 
   uint32_t GetMagicNumber() const { return GetHeaderValue(kMagicNumberOffset); }
+  uint32_t GetExtraReferences() const {
+    return GetHeaderValue(kExtraExternalReferencesOffset);
+  }
 
   class ChunkSizeBits : public BitField<uint32_t, 0, 31> {};
   class IsLastChunkBits : public BitField<bool, 31, 1> {};
 
   static uint32_t ComputeMagicNumber(ExternalReferenceTable* table) {
-    uint32_t external_refs = table->size();
+    uint32_t external_refs = table->size() - table->num_api_references();
     return 0xC0DE0000 ^ external_refs;
   }
+  static uint32_t GetExtraReferences(ExternalReferenceTable* table) {
+    return table->num_api_references();
+  }
 
  protected:
   void SetHeaderValue(int offset, uint32_t value) {
     uint32_t* address = reinterpret_cast<uint32_t*>(data_ + offset);
     memcpy(reinterpret_cast<uint32_t*>(address), &value, sizeof(value));
   }
 
   uint32_t GetHeaderValue(int offset) const {
     uint32_t value;
     memcpy(&value, reinterpret_cast<int*>(data_ + offset), sizeof(value));
     return value;
   }
 
   void AllocateData(int size);
 
   static uint32_t ComputeMagicNumber(Isolate* isolate) {
     return ComputeMagicNumber(ExternalReferenceTable::instance(isolate));
   }
+  static uint32_t GetExtraReferences(Isolate* isolate) {
+    return GetExtraReferences(ExternalReferenceTable::instance(isolate));
+  }
 
   void SetMagicNumber(Isolate* isolate) {
     SetHeaderValue(kMagicNumberOffset, ComputeMagicNumber(isolate));
+    SetHeaderValue(kExtraExternalReferencesOffset, GetExtraReferences(isolate));
   }
 
   static const int kMagicNumberOffset = 0;
+  static const int kExtraExternalReferencesOffset =
+      kMagicNumberOffset + kInt32Size;
 
   byte* data_;
   int size_;
   bool owns_data_;
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_SNAPSHOT_SERIALIZER_COMMON_H_