SnapshotCreator: start from existing snapshot if we have one

src/snapshot/snapshot.h

Index: src/snapshot/snapshot.h
diff --git a/src/snapshot/snapshot.h b/src/snapshot/snapshot.h
index 010072a694f67568f2299452555628d3d26ed30f..a8a1202729cd9509317406be2a9c690ad120929a 100644
--- a/src/snapshot/snapshot.h
+++ b/src/snapshot/snapshot.h
@@ -39,14 +39,14 @@ class SnapshotData : public SerializedData {
   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;
 };