Observe adding external extensions via windows registry.

extensions/browser/external_provider_interface.h

 // Copyright 2013 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 EXTENSIONS_BROWSER_EXTERNAL_PROVIDER_INTERFACE_H_
 #define EXTENSIONS_BROWSER_EXTERNAL_PROVIDER_INTERFACE_H_
 
 #include <vector>
 
 #include "base/memory/linked_ptr.h"
@@ skipping 34 matching lines @@
 
     // Return true if the extension install will proceed.  Install might not
     // proceed if the extension is already installed from a higher priority
     // location.
     virtual bool OnExternalExtensionUpdateUrlFound(
         const std::string& id,
         const std::string& install_parameter,
         const GURL& update_url,
         Manifest::Location location,
         int creation_flags,
-        bool mark_acknowledged) = 0;
+        bool mark_acknowledged,
+        bool is_initial_load) = 0;
 
     // Called after all the external extensions have been reported
     // through the above two methods. |provider| is a pointer to the
     // provider that is now ready (typically this), and the
     // implementation of OnExternalProviderReady() should be able to
     // safely assert that provider->IsReady().
     virtual void OnExternalProviderReady(
         const ExternalProviderInterface* provider) = 0;
 
+    // Once this provider becomes "ready", it can send additional external
+    // extensions it learns about later on through
+    // OnExternalExtensionUpdateUrlFound() or OnExternalExtensionFileFound().
+    // This method will be called each time the provider finds a set of
+    // updated external extensions.
+    virtual void OnExternalProviderUpdateComplete(
+        const ExternalProviderInterface* provider,
+        const std::set<std::string>& removed_extensions) = 0;

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

It seems a little asymmetric that this function gets called with a set of
removed extension ids, but not information about extensions to be added/updated
(I guess those get communicated via calls to
OnExternalExtensionFileFound/OnExternalExtensionUpdateUrlFound prior to this
being called?)

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

Yes.
See my reply in your other comment in ExtensionService.

+
    protected:
     virtual ~VisitorInterface() {}
   };
 
   virtual ~ExternalProviderInterface() {}
 
   // The visitor (ExtensionsService) calls this function before it goes away.
   virtual void ServiceShutdown() = 0;
 
   // Enumerate registered extensions, calling
@@ skipping 17 matching lines @@
   // from its source.
   virtual bool IsReady() const = 0;
 };
 
 typedef std::vector<linked_ptr<ExternalProviderInterface> >
     ProviderCollection;
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_EXTERNAL_PROVIDER_INTERFACE_H_