Replace chrome::NOTIFICATION_EXTENSION_WARNING_CHANGED message with observer pattern

chrome/common/chrome_notification_types.h

Index: chrome/common/chrome_notification_types.h
diff --git a/chrome/common/chrome_notification_types.h b/chrome/common/chrome_notification_types.h
index 103edcf521bb73e6eefe4a0e55db253bbf6b3fb8..25945c4f06597c6908d8cd2d82ab4fc9c06fbd9c 100644
--- a/chrome/common/chrome_notification_types.h
+++ b/chrome/common/chrome_notification_types.h
@@ -683,11 +683,6 @@ enum NotificationType {
   // (const std::string).
   NOTIFICATION_EXTENSION_UPDATE_FOUND,
 
-  // Sent when one or more extensions changed their warning status (like
-  // slowing down Chrome or conflicting with each other).
-  // The source is a Profile.
-  NOTIFICATION_EXTENSION_WARNING_CHANGED,
-
   // An installed app changed notification state (added or removed
   // notifications). The source is a Profile, and the details are a string
   // with the extension id of the app.