Observe adding external extensions via windows registry.

chrome/browser/extensions/external_provider_impl.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_EXTERNAL_PROVIDER_IMPL_H_
 #define CHROME_BROWSER_EXTENSIONS_EXTERNAL_PROVIDER_IMPL_H_
 
 #include <set>
 #include <string>
 
@@ skipping 37 matching lines @@
   // Populates a list with providers for all known sources.
   static void CreateExternalProviders(
       VisitorInterface* service,
       Profile* profile,
       ProviderCollection* provider_list);
 
   // Sets underlying prefs and notifies provider. Only to be called by the
   // 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;
   bool HasExtension(const std::string& id) const override;
   bool GetExtensionDetails(const std::string& id,
                            Manifest::Location* location,
                            scoped_ptr<base::Version>* version) const override;
 
   bool IsReady() const override;
 
@@ skipping 21 matching lines @@
  private:
   bool HandleMinProfileVersion(const base::DictionaryValue* extension,
                                const std::string& extension_id,
                                std::set<std::string>* unsupported_extensions);
 
   bool HandleDoNotInstallForEnterprise(
       const base::DictionaryValue* extension,
       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_;
 
   // Location for external extensions that are provided by this provider from
   // update URLs.
   const Manifest::Location download_location_;
 
   // Weak pointer to the object that consumes the external extensions.
   // This is zeroed out by: ServiceShutdown()
@@ skipping 23 matching lines @@
 
   // Whether the extensions from this provider should be installed immediately.
   bool install_immediately_;
 
   DISALLOW_COPY_AND_ASSIGN(ExternalProviderImpl);
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_EXTERNAL_PROVIDER_IMPL_H_