Fix classes that have too many virtuals for inline constructors.

components/mus/public/cpp/window_observer.h

 // Copyright 2014 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 COMPONENTS_MUS_PUBLIC_CPP_WINDOW_OBSERVER_H_
 #define COMPONENTS_MUS_PUBLIC_CPP_WINDOW_OBSERVER_H_
 
 #include <vector>
 
 #include "components/mus/public/cpp/window.h"
@@ skipping 79 matching lines @@
                                             intptr_t old) {}
 
   virtual void OnWindowEmbeddedAppDisconnected(Window* window) {}
 
   // Sent when the drawn state changes. This is only sent for the root nodes
   // when embedded.
   virtual void OnWindowDrawnChanging(Window* window) {}
   virtual void OnWindowDrawnChanged(Window* window) {}
 
  protected:
+  WindowObserver() = default;
   virtual ~WindowObserver() {}
 };
 
 }  // namespace mus
 
 #endif  // COMPONENTS_MUS_PUBLIC_CPP_WINDOW_OBSERVER_H_