WidgetDelegateView is its own contents view.

ui/views/widget/widget_delegate.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_VIEWS_WIDGET_WIDGET_DELEGATE_H_
 #define UI_VIEWS_WIDGET_WIDGET_DELEGATE_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 193 matching lines @@
   // Internal class name.
   static const char kViewClassName[];
 
   WidgetDelegateView();
   ~WidgetDelegateView() override;
 
   // WidgetDelegate:
   void DeleteDelegate() override;
   Widget* GetWidget() override;
   const Widget* GetWidget() const override;
+  views::View* GetContentsView() override;
 
   // View:
   const char* GetClassName() const override;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(WidgetDelegateView);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_WIDGET_WIDGET_DELEGATE_H_