Fix classes that have too many virtuals for inline constructors.

ui/views/widget/widget_observer.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_OBSERVER_H_
 #define UI_VIEWS_WIDGET_WIDGET_OBSERVER_H_
 
 #include "ui/views/views_export.h"
 
 namespace gfx {
@@ skipping 29 matching lines @@
   virtual void OnWidgetVisibilityChanging(Widget* widget, bool visible) {}
 
   virtual void OnWidgetVisibilityChanged(Widget* widget, bool visible) {}
 
   virtual void OnWidgetActivationChanged(Widget* widget, bool active) {}
 
   virtual void OnWidgetBoundsChanged(Widget* widget,
                                      const gfx::Rect& new_bounds) {}
 
  protected:
+  WidgetObserver() = default;
   virtual ~WidgetObserver() {}
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_WIDGET_WIDGET_OBSERVER_H_