SnapshotCreator: start from existing snapshot if we have one

src/snapshot/serializer-common.h

Index: src/snapshot/serializer-common.h
diff --git a/src/snapshot/serializer-common.h b/src/snapshot/serializer-common.h
index b426efd538e99384dea8f8088ee61094b154752e..dfe58245b4e9b26780f2f7b704d5476edd7aafc5 100644
--- a/src/snapshot/serializer-common.h
+++ b/src/snapshot/serializer-common.h
@@ -245,14 +245,20 @@ class SerializedData {
   }
 
   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) {
@@ -271,12 +277,18 @@ class SerializedData {
   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_;