Remove NOTIFICATION_EXTENSION_ENABLED.

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 a7a9e9bfff37ce8530c6a30df92cf93d273b12fa..a067b36d844b8c8f0d2a6337fd2a9039a90de006 100644
--- a/extensions/browser/extension_registry_observer.h
+++ b/extensions/browser/extension_registry_observer.h
@@ -29,6 +29,12 @@ class ExtensionRegistryObserver {
       content::BrowserContext* browser_context,
       const Extension* extension) {}
 
+  // Called when an extension is enabled. Under most circumstances, listeners
+  // will want to use OnExtensionLoaded(). This is only called when the "Enable"
+  // button is hit in the extensions tab.
+  virtual void OnExtensionEnabled(content::BrowserContext* browser_context,
+                                  const Extension* extension) {}
+
   // Called after an extension is loaded and all necessary browser state is
   // initialized to support the start of the extension's child process.
   virtual void OnExtensionReady(content::BrowserContext* browser_context,