Observe adding external extensions via windows registry.

chrome/browser/extensions/extension_service.h

Index: chrome/browser/extensions/extension_service.h
diff --git a/chrome/browser/extensions/extension_service.h b/chrome/browser/extensions/extension_service.h
index 01fea8ec5a874d8dd5078d7d1009d18ce2fd092e..e847253ff416c84190135ec7b6883c2ae4bab60c 100644
--- a/chrome/browser/extensions/extension_service.h
+++ b/chrome/browser/extensions/extension_service.h
@@ -228,7 +228,7 @@ class ExtensionService
   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,
@@ -243,9 +243,13 @@ class ExtensionService
       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;