Fix classes that have too many virtuals for inline constructors.

ui/message_center/message_center_observer.h

Index: ui/message_center/message_center_observer.h
diff --git a/ui/message_center/message_center_observer.h b/ui/message_center/message_center_observer.h
index 5ab0722be13c1e9323085dcd8bf4c6743837c74f..6c9790a42f7678d9f0c0122feac16d1ed61391fa 100644
--- a/ui/message_center/message_center_observer.h
+++ b/ui/message_center/message_center_observer.h
@@ -58,6 +58,9 @@ class MESSAGE_CENTER_EXPORT MessageCenterObserver {
 
   // Called when the blocking state of |blocker| is changed.
   virtual void OnBlockingStateChanged(NotificationBlocker* blocker) {}
+
+ protected:
+  MessageCenterObserver() = default;
 };
 
 }  // namespace message_center