[serializer] print use count of external references.

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; }
 
+#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);
+
   static const int kDeoptTableSerializeEntryCount = 64;
 
  private:
   struct ExternalReferenceEntry {
     Address address;
     const char* name;
+#ifdef DEBUG
+    int count;
+#endif  // DEBUG
   };
 
   explicit ExternalReferenceTable(Isolate* isolate);
 
   void Add(Address address, const char* name) {
+#ifdef DEBUG
+    ExternalReferenceEntry entry = {address, name, 0};
+#else
     ExternalReferenceEntry entry = {address, name};
+#endif  // DEBUG
     refs_.Add(entry);
   }
 
   void AddReferences(Isolate* isolate);
   void AddBuiltins(Isolate* isolate);
   void AddRuntimeFunctions(Isolate* isolate);
   void AddIsolateAddresses(Isolate* isolate);
   void AddAccessors(Isolate* isolate);
   void AddStubCache(Isolate* isolate);
   void AddDeoptEntries(Isolate* isolate);
   void AddApiReferences(Isolate* isolate);
 
   List<ExternalReferenceEntry> refs_;
 
   DISALLOW_COPY_AND_ASSIGN(ExternalReferenceTable);
 };
 
 }  // namespace internal
 }  // namespace v8
 #endif  // V8_EXTERNAL_REFERENCE_TABLE_H_