Fix classes that have too many virtuals for inline constructors.

extensions/browser/extension_registry_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 EXTENSIONS_BROWSER_EXTENSION_REGISTRY_OBSERVER_H_
 #define EXTENSIONS_BROWSER_EXTENSION_REGISTRY_OBSERVER_H_
 
 #include "extensions/browser/uninstall_reason.h"
 #include "extensions/common/extension.h"
 
@@ skipping 58 matching lines @@
                                     bool is_update) {}
 
   // Called after an extension is uninstalled. The extension no longer exists in
   // any of the ExtensionRegistry sets (enabled, disabled, etc.).
   virtual void OnExtensionUninstalled(content::BrowserContext* browser_context,
                                       const Extension* extension,
                                       UninstallReason reason) {}
 
   // Notifies observers that the observed object is going away.
   virtual void OnShutdown(ExtensionRegistry* registry) {}
+
+ protected:
+  ExtensionRegistryObserver() = default;
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_EXTENSION_REGISTRY_OBSERVER_H_