SnapshotCreator: start from existing snapshot if we have one

src/external-reference-table.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_EXTERNAL_REFERENCE_TABLE_H_
 #define V8_EXTERNAL_REFERENCE_TABLE_H_
 
 #include "src/address-map.h"
 
 namespace v8 {
 namespace internal {
 
 class Isolate;
 
 // ExternalReferenceTable is a helper class that defines the relationship
 // between external references and their encodings. It is used to build
 // hashmaps in ExternalReferenceEncoder and ExternalReferenceDecoder.
 class ExternalReferenceTable {
  public:
   static ExternalReferenceTable* instance(Isolate* isolate);
 
   uint32_t size() const { return static_cast<uint32_t>(refs_.length()); }
   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++; }
   int count(uint32_t i) { return refs_[i].count; }
   void ResetCount();
   void PrintCount();
 #endif  // DEBUG
 
   static const char* ResolveSymbol(void* address);
 
@@ skipping 30 matching lines @@
 
   List<ExternalReferenceEntry> refs_;
   uint32_t api_refs_start_;
 
   DISALLOW_COPY_AND_ASSIGN(ExternalReferenceTable);
 };
 
 }  // namespace internal
 }  // namespace v8
 #endif  // V8_EXTERNAL_REFERENCE_TABLE_H_