First cut at the component observer.

chrome/browser/chrome_notification_types.h

 // Copyright 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 CHROME_BROWSER_CHROME_NOTIFICATION_TYPES_H_
 #define CHROME_BROWSER_CHROME_NOTIFICATION_TYPES_H_
 
 #include "build/build_config.h"
 #include "content/public/browser/notification_types.h"
 
@@ skipping 652 matching lines @@
   // extensions. The source is a Profile, and there are no details.
   NOTIFICATION_EXTENSION_UPDATING_STARTED,
 
   // The extension updater found an update and will attempt to download and
   // install it. The source is a Profile, and the details are an
   // extensions::UpdateDetails object with the extension id and version of the
   // found update.
   NOTIFICATION_EXTENSION_UPDATE_FOUND,
 
   // Component Updater -------------------------------------------------------
-
+  //
+  // The component updater notifications are deprecated and they will be
+  // removed soon.
+  //
   // Sent when the component updater starts doing update checks. If no
   // component has been registered for update this notification is not
   // generated. The source is the component updater itself and there are
   // no details.
   NOTIFICATION_COMPONENT_UPDATER_STARTED,
 
   // Sent when the component updater is going to take a long nap. The
   // source is the component updater itself and there are no details.
   NOTIFICATION_COMPONENT_UPDATER_SLEEPING,
 
@@ skipping 552 matching lines @@
   // Note:-
   // Currently only Content and Chrome define and use notifications.
   // Custom notifications not belonging to Content and Chrome should start
   // from here.
   NOTIFICATION_CHROME_END,
 };
 
 }  // namespace chrome
 
 #endif  // CHROME_BROWSER_CHROME_NOTIFICATION_TYPES_H_