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..6531e3478c9800787f1926bfc0a9a6f17e19012f 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 +100,8 @@ class ExternalProviderImpl : public ExternalProviderInterface {
       const std::string& extension_id,
       std::set<std::string>* unsupported_extensions);
 
+  void NotifyExtensionsFromPrefs(bool is_initial_load);

[asargent_no_longer_on_chrome, 2016/01/20 22:49:00]

nit: this could use an overview comment

[lazyboy, 2016/01/21 21:02:12]

Done.

+
   // Location for external extensions that are provided by this provider from
   // local crx files.
   const Manifest::Location crx_location_;