[serializer] print use count of external references.

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 fb671a86307d285903cb21f728c5629e0db0f279..e1b97f957c32fca8bd567315523cbb2d2784af3b 100644
--- a/src/external-reference-table.h
+++ b/src/external-reference-table.h
@@ -23,18 +23,34 @@ class ExternalReferenceTable {
   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);
   }