[serializer] allow duplicate API external references.

test/cctest/test-serialize.cc

 // Copyright 2007-2010 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1968 matching lines @@
     args.GetReturnValue().Set(v8_num(12));
   }
 };
 
 intptr_t original_external_references[] = {
     reinterpret_cast<intptr_t>(SerializedCallback),
     reinterpret_cast<intptr_t>(&serialized_static_field),
     reinterpret_cast<intptr_t>(&NamedPropertyGetterForSerialization),
     reinterpret_cast<intptr_t>(&AccessorForSerialization),
     reinterpret_cast<intptr_t>(&SerializedExtension::FunctionCallback),
+    reinterpret_cast<intptr_t>(&serialized_static_field),  // duplicate entry
     0};
 
 intptr_t replaced_external_references[] = {
     reinterpret_cast<intptr_t>(SerializedCallbackReplacement),
     reinterpret_cast<intptr_t>(&serialized_static_field),
     reinterpret_cast<intptr_t>(&NamedPropertyGetterForSerialization),
     reinterpret_cast<intptr_t>(&AccessorForSerialization),
     reinterpret_cast<intptr_t>(&SerializedExtension::FunctionCallback),
+    reinterpret_cast<intptr_t>(&serialized_static_field),
     0};
 
 TEST(SnapshotCreatorExternalReferences) {
   DisableAlwaysOpt();
   v8::StartupData blob;
   {
     v8::SnapshotCreator creator(original_external_references);
     v8::Isolate* isolate = creator.GetIsolate();
     {
       v8::HandleScope handle_scope(isolate);
@@ skipping 448 matching lines @@
   }
   delete[] blob.data;
 }
 
 TEST(SerializationMemoryStats) {
   FLAG_profile_deserialization = true;
   FLAG_always_opt = false;
   v8::StartupData blob = v8::V8::CreateSnapshotDataBlob();
   delete[] blob.data;
 }