Improve TypeError messages from failed array conversions.

Source/bindings/v8/custom/V8DedicatedWorkerGlobalScopeCustom.cpp

 /*
  * Copyright (C) 2009, 2011 Google Inc. 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
@@ skipping 29 matching lines @@
 #include "wtf/ArrayBuffer.h"
 
 namespace WebCore {
 
 void V8DedicatedWorkerGlobalScope::postMessageMethodCustom(const v8::FunctionCallbackInfo<v8::Value>& args)
 {
     DedicatedWorkerGlobalScope* workerGlobalScope = V8DedicatedWorkerGlobalScope::toNative(args.Holder());
     MessagePortArray ports;
     ArrayBufferArray arrayBuffers;
     if (args.Length() > 1) {
+        const int transferablesArgIndex = 1;
         bool notASequence = false;
-        if (!extractTransferables(args[1], ports, arrayBuffers, notASequence, args.GetIsolate())) {
+        if (!extractTransferables(args[transferablesArgIndex], ports, arrayBuffers, notASequence, args.GetIsolate())) {
             if (notASequence)
-                throwTypeError(ExceptionMessages::failedToExecute("postMessage", "WorkerGlobalScope", ExceptionMessages::notASequenceType("Second")), args.GetIsolate());
+                throwTypeError(ExceptionMessages::failedToExecute("postMessage", "WorkerGlobalScope", ExceptionMessages::notASequenceTypeArgumentOrValue(transferablesArgIndex + 1)), args.GetIsolate());
             return;
         }
     }
     bool didThrow = false;
     RefPtr<SerializedScriptValue> message =
         SerializedScriptValue::create(args[0],
                                       &ports,
                                       &arrayBuffers,
                                       didThrow,
                                       args.GetIsolate());
     if (didThrow)
         return;
     ExceptionState es(args.GetIsolate());
     workerGlobalScope->postMessage(message.release(), &ports, es);
     es.throwIfNeeded();
 }
 
 } // namespace WebCore