Fix classes that have too many virtuals for inline constructors.

ui/message_center/message_center_observer.h

 // Copyright (c) 2013 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_MESSAGE_CENTER_MESSAGE_CENTER_OBSERVER_H_
 #define UI_MESSAGE_CENTER_MESSAGE_CENTER_OBSERVER_H_
 
 #include <string>
 
 #include "ui/message_center/message_center_export.h"
@@ skipping 40 matching lines @@
 
   // Called when the notification center is shown or hidden.
   virtual void OnCenterVisibilityChanged(Visibility visibility) {}
 
   // Called whenever the quiet mode changes as a result of user action or when
   // quiet mode expires.
   virtual void OnQuietModeChanged(bool in_quiet_mode) {}
 
   // Called when the blocking state of |blocker| is changed.
   virtual void OnBlockingStateChanged(NotificationBlocker* blocker) {}
+
+ protected:
+  MessageCenterObserver() = default;
 };
 
 }  // namespace message_center
 
 #endif  // UI_MESSAGE_CENTER_MESSAGE_CENTER_OBSERVER_H_