Fix classes that have too many virtuals for inline constructors.

extensions/browser/extension_registry_observer.h

Index: extensions/browser/extension_registry_observer.h
diff --git a/extensions/browser/extension_registry_observer.h b/extensions/browser/extension_registry_observer.h
index 2b984651e18314c47c4d22fca3008bcae820cf60..84eda52f17ee4f8fdd2d8c8c5f53a92fedec69d1 100644
--- a/extensions/browser/extension_registry_observer.h
+++ b/extensions/browser/extension_registry_observer.h
@@ -76,6 +76,9 @@ class ExtensionRegistryObserver {
 
   // Notifies observers that the observed object is going away.
   virtual void OnShutdown(ExtensionRegistry* registry) {}
+
+ protected:
+  ExtensionRegistryObserver() = default;
 };
 
 }  // namespace extensions