Have postMessage()s throw TypeError on failed arity checks.

Source/bindings/core/v8/custom/V8WindowCustom.cpp

Index: Source/bindings/core/v8/custom/V8WindowCustom.cpp
diff --git a/Source/bindings/core/v8/custom/V8WindowCustom.cpp b/Source/bindings/core/v8/custom/V8WindowCustom.cpp
index 4a8d935ab28e4d63cfd619b54e033b0cebc8c6b1..cd38053741c40b59e361d5264c23dcdc410cc6e8 100644
--- a/Source/bindings/core/v8/custom/V8WindowCustom.cpp
+++ b/Source/bindings/core/v8/custom/V8WindowCustom.cpp
@@ -159,6 +159,13 @@ static bool isLegacyTargetOriginDesignation(v8::Handle<v8::Value> value)
 
 void V8Window::postMessageMethodCustom(const v8::FunctionCallbackInfo<v8::Value>& info)
 {
+    ExceptionState exceptionState(ExceptionState::ExecutionContext, "postMessage", "Window", info.Holder(), info.GetIsolate());
+    if (UNLIKELY(info.Length() < 2)) {
+        setMinimumArityTypeError(exceptionState, 2, info.Length());
+        exceptionState.throwIfNeeded();
+        return;
+    }
+
     // None of these need to be RefPtr because info and context are guaranteed
     // to hold on to them.
     DOMWindow* window = V8Window::toImpl(info.Holder());
@@ -167,8 +174,6 @@ void V8Window::postMessageMethodCustom(const v8::FunctionCallbackInfo<v8::Value>
     ASSERT(window);
     UseCounter::countIfNotPrivateScript(info.GetIsolate(), window->frame(), UseCounter::WindowPostMessage);
 
-    ExceptionState exceptionState(ExceptionState::ExecutionContext, "postMessage", "Window", info.Holder(), info.GetIsolate());
-
     // If called directly by WebCore we don't have a calling context.
     if (!source) {
         exceptionState.throwTypeError("No active calling context exists.");