SnapshotCreator: start from existing snapshot if we have one

src/snapshot/deserializer.h

Index: src/snapshot/deserializer.h
diff --git a/src/snapshot/deserializer.h b/src/snapshot/deserializer.h
index 0348956eb6860dd74fa75e9ef60fe6d25f543c5a..35d2d5ea7892a1ffc1e8b905135ac3aa6412a5fd 100644
--- a/src/snapshot/deserializer.h
+++ b/src/snapshot/deserializer.h
@@ -34,6 +34,7 @@ class Deserializer : public SerializerDeserializer {
       : 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),
@@ -125,6 +126,7 @@ class Deserializer : public SerializerDeserializer {
 
   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