Observe adding external extensions via windows registry.

chrome/browser/extensions/external_provider_impl.h

Index: chrome/browser/extensions/external_provider_impl.h
diff --git a/chrome/browser/extensions/external_provider_impl.h b/chrome/browser/extensions/external_provider_impl.h
index 2d816bc04e1c64c000b12f4f451e1a8c6e0fc1f1..68083f215002a3cc3197ec34f7901b2e5038f42f 100644
--- a/chrome/browser/extensions/external_provider_impl.h
+++ b/chrome/browser/extensions/external_provider_impl.h
@@ -55,6 +55,10 @@ class ExternalProviderImpl : public ExternalProviderInterface {
   // owned ExternalLoader instance.
   virtual void SetPrefs(base::DictionaryValue* prefs);
 
+  // Updates the underlying prefs and notifies provider.
+  // Only to be called by the owned ExternalLoader instance.
+  void UpdatePrefs(base::DictionaryValue* prefs);
+
   // ExternalProvider implementation:
   void ServiceShutdown() override;
   void VisitRegisteredExtension() override;
@@ -96,6 +100,14 @@ class ExternalProviderImpl : public ExternalProviderInterface {
       const std::string& extension_id,
       std::set<std::string>* unsupported_extensions);
 
+  // Notifies our consumer about the extensions that were found in this
+  // provider.
+  // |is_initial_load| indicates whether or not we are sending the notification
+  // for the first time. Generally, a provider only sends this notification
+  // once, but it can choose to provide further notifications. Provider with
+  // ExternalRegistryLoader is an example of such.
+  void NotifyExtensionsFromPrefs(bool is_initial_load);
+
   // Location for external extensions that are provided by this provider from
   // local crx files.
   const Manifest::Location crx_location_;