SnapshotCreator: start from existing snapshot if we have one

src/external-reference-table.h

Index: src/external-reference-table.h
diff --git a/src/external-reference-table.h b/src/external-reference-table.h
index 40eccbedf035f38d580e9e5a22726acd0264af0f..4b167cec721ebc6535b04411c98dbc922a4839eb 100644
--- a/src/external-reference-table.h
+++ b/src/external-reference-table.h
@@ -23,6 +23,7 @@ class ExternalReferenceTable {
   Address address(uint32_t i) { return refs_[i].address; }
   const char* name(uint32_t i) { return refs_[i].name; }
   bool is_api_reference(uint32_t i) { return i >= api_refs_start_; }
+  uint32_t num_api_references() { return size() - api_refs_start_; }
 
 #ifdef DEBUG
   void increment_count(uint32_t i) { refs_[i].count++; }