Simplify the GuestWindowManager

Instead of recording guest explicitly in a map, we just make use of
closure for bookkeeping.

atom/browser/api/atom_api_window.h

@@ -59,6 +59,7 @@ class Window : public mate::EventEmitter,
   // APIs for NativeWindow.
   void Destroy();
   void Close();
+  bool IsClosed();
   void Focus();
   bool IsFocused();
   void Show();