Remove InstallTracker's extension load, unload observer

extensions/browser/extension_registry_observer.h

 // Copyright 2014 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 EXTENSIONS_BROWSER_EXTENSION_REGISTRY_OBSERVER_H_
 #define EXTENSIONS_BROWSER_EXTENSION_REGISTRY_OBSERVER_H_
 
 #include "extensions/common/extension.h"
 
 namespace content {
@@ skipping 44 matching lines @@
       content::BrowserContext* browser_context,
       const Extension* extension,
       bool is_update,
       bool from_ephemeral,
       const std::string& old_name) {}
 
   // Called after an extension is uninstalled. The extension no longer exsit in
   // any of the ExtensionRegistry sets (enabled, disabled, etc.).
   virtual void OnExtensionUninstalled(content::BrowserContext* browser_context,
                                       const Extension* extension) {}
+
+  // Notifies observers that the observed object is going away.
+  virtual void OnShutdown() {}
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_EXTENSION_REGISTRY_OBSERVER_H_