Revert of Make Window.prototype.toString return [object Window] at all times.

Source/bindings/core/v8/custom/V8WindowCustom.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 200 matching lines @@
     TOSTRING_VOID(V8StringResource<TreatNullAndUndefinedAsNullString>, targetOrigin, info[targetOriginArgIndex]);
 
     RefPtr<SerializedScriptValue> message = SerializedScriptValueFactory::instance().create(info.GetIsolate(), info[0], &portArray, &arrayBufferArray, exceptionState);
     if (exceptionState.throwIfNeeded())
         return;
 
     window->postMessage(message.release(), &portArray, targetOrigin, source, exceptionState);
     exceptionState.throwIfNeeded();
 }
 
-// TODO(arv): Remove this custom method and implement Symbol.toStringTag instead.
+// FIXME(fqian): returning string is cheating, and we should
+// fix this by calling toString function on the receiver.
+// However, V8 implements toString in JavaScript, which requires
+// switching context of receiver. I consider it is dangerous.
 void V8Window::toStringMethodCustom(const v8::FunctionCallbackInfo<v8::Value>& info)
 {
-    v8SetReturnValueString(info, "[object Window]", info.GetIsolate());
+    v8::Local<v8::Object> domWrapper = V8Window::findInstanceInPrototypeChain(info.This(), info.GetIsolate());
+    v8::Local<v8::Object> target = domWrapper.IsEmpty() ? info.This() : domWrapper;
+    v8::Local<v8::String> value;
+    if (target->ObjectProtoToString(info.GetIsolate()->GetCurrentContext()).ToLocal(&value))
+        v8SetReturnValue(info, value);
 }
 
 void V8Window::openMethodCustom(const v8::FunctionCallbackInfo<v8::Value>& info)
 {
     LocalDOMWindow* impl = toLocalDOMWindow(V8Window::toImpl(info.Holder()));
     ExceptionState exceptionState(ExceptionState::ExecutionContext, "open", "Window", info.Holder(), info.GetIsolate());
     if (!BindingSecurity::shouldAllowAccessToFrame(info.GetIsolate(), impl->frame(), exceptionState)) {
         exceptionState.throwIfNeeded();
         return;
     }
@@ skipping 103 matching lines @@
 {
     return securityCheck(host);
 }
 
 bool V8Window::indexedSecurityCheckCustom(v8::Local<v8::Object> host, uint32_t index, v8::AccessType type, v8::Local<v8::Value>)
 {
     return securityCheck(host);
 }
 
 } // namespace blink