Revert of Remove DialogDelegate::OnClosed() which is redundant with

ui/views/window/dialog_delegate.h

Index: ui/views/window/dialog_delegate.h
diff --git a/ui/views/window/dialog_delegate.h b/ui/views/window/dialog_delegate.h
index 68b1f202e2f95118126092ee4f0ff935faec4f47..dc9f89a1aac7501fb8e8b77142ad013d864f2741 100644
--- a/ui/views/window/dialog_delegate.h
+++ b/ui/views/window/dialog_delegate.h
@@ -109,6 +109,9 @@
   // Returns whether this particular dialog should use the new dialog style.
   virtual bool UseNewStyleForThisDialog() const;
 
+  // Called when the window has been closed.
+  virtual void OnClosed() {}
+
   // A helper for accessing the DialogClientView object contained by this
   // delegate's Window.
   const DialogClientView* GetDialogClientView() const;