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 {
 class BrowserContext;
 }
 
 namespace extensions {
 
 class Extension;
+class ExtensionRegistry;
 struct UnloadedExtensionInfo;
 
 // Observer for ExtensionRegistry. Exists in a separate header file to reduce
 // the include file burden for typical clients of ExtensionRegistry.
 class ExtensionRegistryObserver {
  public:
   virtual ~ExtensionRegistryObserver() {}
 
   // Called after an extension is loaded. The extension will exclusively exist
   // in the enabled_extensions set of ExtensionRegistry.
@@ skipping 28 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(ExtensionRegistry* registry) {}
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_EXTENSION_REGISTRY_OBSERVER_H_