[serializer] print use count of external references.

src/api.cc

 // Copyright 2012 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.
 
 #include "src/api.h"
 
 #include <string.h>  // For memcpy, strlen.
 #ifdef V8_USE_ADDRESS_SANITIZER
 #include <sanitizer/asan_interface.h>
 #endif  // V8_USE_ADDRESS_SANITIZER
@@ skipping 541 matching lines @@
   int num_contexts = static_cast<int>(data->contexts_.Size());
   i::List<i::Object*> contexts(num_contexts);
   for (int i = 0; i < num_contexts; i++) {
     i::HandleScope scope(isolate);
     i::Handle<i::Context> context =
         v8::Utils::OpenHandle(*data->contexts_.Get(i));
     contexts.Add(*context);
   }
   data->contexts_.Clear();
 
+#ifdef DEBUG
+  i::ExternalReferenceTable::instance(isolate)->ResetCount();
+#endif  // DEBUG
+
   i::StartupSerializer startup_serializer(isolate, function_code_handling);
   startup_serializer.SerializeStrongReferences();
 
   // Serialize each context with a new partial serializer.
   i::List<i::SnapshotData*> context_snapshots(num_contexts);
   for (int i = 0; i < num_contexts; i++) {
     i::PartialSerializer partial_serializer(isolate, &startup_serializer,
                                             callback);
     partial_serializer.Serialize(&contexts[i]);
     context_snapshots.Add(new i::SnapshotData(&partial_serializer));
   }
 
   startup_serializer.SerializeWeakReferencesAndDeferred();
+
+#ifdef DEBUG
+  if (i::FLAG_external_reference_stats) {
+    i::ExternalReferenceTable::instance(isolate)->PrintCount();
+  }
+#endif  // DEBUG
+
   i::SnapshotData startup_snapshot(&startup_serializer);
   StartupData result =
       i::Snapshot::CreateSnapshotBlob(&startup_snapshot, &context_snapshots);
 
   // Delete heap-allocated context snapshot instances.
   for (const auto& context_snapshot : context_snapshots) {
     delete context_snapshot;
   }
   data->created_ = true;
   return result;
@@ skipping 9188 matching lines @@
   Address callback_address =
       reinterpret_cast<Address>(reinterpret_cast<intptr_t>(callback));
   VMState<EXTERNAL> state(isolate);
   ExternalCallbackScope call_scope(isolate, callback_address);
   callback(info);
 }
 
 
 }  // namespace internal
 }  // namespace v8