[serializer] allow duplicate API external references.

src/external-reference-table.cc

 // 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.
 
 #include "src/external-reference-table.h"
 
 #include "src/accessors.h"
 #include "src/assembler.h"
 #include "src/builtins/builtins.h"
 #include "src/counters.h"
@@ skipping 425 matching lines @@
     Address address = Deoptimizer::GetDeoptimizationEntry(
         isolate, entry, Deoptimizer::LAZY,
         Deoptimizer::CALCULATE_ENTRY_ADDRESS);
     Add(address, "lazy_deopt");
   }
 }
 
 void ExternalReferenceTable::AddApiReferences(Isolate* isolate) {
   // Add external references provided by the embedder (a null-terminated
   // array).
+  api_refs_start_ = size();
   intptr_t* api_external_references = isolate->api_external_references();
   if (api_external_references != nullptr) {
     while (*api_external_references != 0) {
       Address address = reinterpret_cast<Address>(*api_external_references);
       Add(address, ResolveSymbol(address));
       api_external_references++;
     }
   }
 }
 
 }  // namespace internal
 }  // namespace v8