[serializer] print use count of external references.

src/snapshot/serializer-common.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_SNAPSHOT_SERIALIZER_COMMON_H_
 #define V8_SNAPSHOT_SERIALIZER_COMMON_H_
 
 #include "src/address-map.h"
 #include "src/external-reference-table.h"
 #include "src/globals.h"
 
 namespace v8 {
 namespace internal {
 
 class Isolate;
 
 class ExternalReferenceEncoder {
  public:
   explicit ExternalReferenceEncoder(Isolate* isolate);
 
   uint32_t Encode(Address key) const;
 
   const char* NameOfAddress(Isolate* isolate, Address address) const;
 
  private:
   AddressToIndexHashMap* map_;
+#ifdef DEBUG
+  ExternalReferenceTable* table_;
+#endif  // DEBUG
 
   DISALLOW_COPY_AND_ASSIGN(ExternalReferenceEncoder);
 };
 
 class HotObjectsList {
  public:
   HotObjectsList() : index_(0) {
     for (int i = 0; i < kSize; i++) circular_queue_[i] = NULL;
   }
 
@@ skipping 238 matching lines @@
 
   byte* data_;
   int size_;
   bool owns_data_;
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_SNAPSHOT_SERIALIZER_COMMON_H_