Improve TypeError messages from failed array conversions.

Source/bindings/v8/custom/V8DedicatedWorkerGlobalScopeCustom.cpp

Index: Source/bindings/v8/custom/V8DedicatedWorkerGlobalScopeCustom.cpp
diff --git a/Source/bindings/v8/custom/V8DedicatedWorkerGlobalScopeCustom.cpp b/Source/bindings/v8/custom/V8DedicatedWorkerGlobalScopeCustom.cpp
index 8994365f8878950e09fbbe3434e2b09a9ff7d5a2..5ea4c74ff328729c16bfc840113f5f73d11a6685 100644
--- a/Source/bindings/v8/custom/V8DedicatedWorkerGlobalScopeCustom.cpp
+++ b/Source/bindings/v8/custom/V8DedicatedWorkerGlobalScopeCustom.cpp
@@ -47,10 +47,11 @@ void V8DedicatedWorkerGlobalScope::postMessageMethodCustom(const v8::FunctionCal
     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;
         }
     }