Improve TypeError messages from failed array conversions.

Source/bindings/v8/custom/V8WorkerCustom.cpp

Index: Source/bindings/v8/custom/V8WorkerCustom.cpp
diff --git a/Source/bindings/v8/custom/V8WorkerCustom.cpp b/Source/bindings/v8/custom/V8WorkerCustom.cpp
index d4d6a9f9c3caa39646cadc2aa2b036f4ddcb994a..2a5b62632ca81cb5632f80ca201c7127aef1298f 100644
--- a/Source/bindings/v8/custom/V8WorkerCustom.cpp
+++ b/Source/bindings/v8/custom/V8WorkerCustom.cpp
@@ -48,10 +48,11 @@ void V8Worker::postMessageMethodCustom(const v8::FunctionCallbackInfo<v8::Value>
     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", "Worker", ExceptionMessages::notASequenceType("Second")), args.GetIsolate());
+                throwTypeError(ExceptionMessages::failedToExecute("postMessage", "Worker", ExceptionMessages::notASequenceTypeArgumentOrValue(transferablesArgIndex + 1)), args.GetIsolate());
             return;
         }
     }