Fix classes that have too many virtuals for inline constructors.

ui/app_list/app_list_model_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_APP_LIST_APP_LIST_MODEL_OBSERVER_H_
 #define UI_APP_LIST_APP_LIST_MODEL_OBSERVER_H_
 
 #include "ui/app_list/app_list_export.h"
 #include "ui/app_list/app_list_model.h"
 
@@ skipping 22 matching lines @@
   // Triggered after |item| has moved, changed folders, or changed properties.
   virtual void OnAppListItemUpdated(AppListItem* item) {}
 
   // Triggered when the custom launcher page enabled state is changed.
   virtual void OnCustomLauncherPageEnabledStateChanged(bool enabled) {}
 
   // Triggered when the search engine is changed to and from Google.
   virtual void OnSearchEngineIsGoogleChanged(bool is_google) {}
 
  protected:
+  AppListModelObserver() = default;
   virtual ~AppListModelObserver() {}
 };
 
 }  // namespace app_list
 
 #endif  // UI_APP_LIST_APP_LIST_MODEL_OBSERVER_H_