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

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 5af2203cbf40bc00d14d517ac9ff96c88dea094e..f2d3ecc95104851b0406ae4abc62cb7a0a550285 100644
--- a/Source/bindings/core/v8/custom/V8WindowCustom.cpp
+++ b/Source/bindings/core/v8/custom/V8WindowCustom.cpp
@@ -218,10 +218,17 @@
     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)