Observe adding external extensions via windows registry.

chrome/browser/extensions/extension_service.h

 // Copyright (c) 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_EXTENSIONS_EXTENSION_SERVICE_H_
 #define CHROME_BROWSER_EXTENSIONS_EXTENSION_SERVICE_H_
 
 #include <map>
 #include <set>
 #include <string>
@@ skipping 210 matching lines @@
   void AddExtension(const extensions::Extension* extension) override;
   void AddComponentExtension(const extensions::Extension* extension) override;
   const extensions::Extension* GetPendingExtensionUpdate(
       const std::string& extension_id) const override;
   void FinishDelayedInstallation(const std::string& extension_id) override;
   void CheckManagementPolicy() override;
   void CheckForUpdatesSoon() override;
   bool is_ready() override;
   base::SequencedTaskRunner* GetFileTaskRunner() override;
 
-  // ExternalProvider::Visitor implementation.
+  // ExternalProvider::VisitorInterface implementation.
   // Exposed for testing.
   bool OnExternalExtensionFileFound(const std::string& id,
                                     const base::Version* version,
                                     const base::FilePath& path,
                                     extensions::Manifest::Location location,
                                     int creation_flags,
                                     bool mark_acknowledged,
                                     bool install_immediately) override;
   bool OnExternalExtensionUpdateUrlFound(
       const std::string& id,
       const std::string& install_parameter,
       const GURL& update_url,
       extensions::Manifest::Location location,
       int creation_flags,
-      bool mark_acknowledged) override;
+      bool mark_acknowledged,
+      bool is_initial_load) override;
   void OnExternalProviderReady(
       const extensions::ExternalProviderInterface* provider) override;
+  void OnExternalProviderUpdateComplete(
+      const extensions::ExternalProviderInterface* provider,
+      const std::set<std::string>& removed_extensions) override;
 
   // ExtensionManagement::Observer implementation:
   void OnExtensionManagementSettingsChanged() override;
 
   // Initialize and start all installed extensions.
   void Init();
 
   // Called when the associated Profile is going to be destroyed.
   void Shutdown();
 
@@ skipping 494 matching lines @@
                            GreylistUnknownDontChange);
   FRIEND_TEST_ALL_PREFIXES(ExtensionServiceTest,
                            ManagementPolicyProhibitsEnableOnInstalled);
   FRIEND_TEST_ALL_PREFIXES(ExtensionServiceTest,
                            BlockAndUnblockBlacklistedExtension);
 
   DISALLOW_COPY_AND_ASSIGN(ExtensionService);
 };
 
 #endif  // CHROME_BROWSER_EXTENSIONS_EXTENSION_SERVICE_H_