Observe adding external extensions via windows registry.

extensions/browser/external_provider_interface.h

Index: extensions/browser/external_provider_interface.h
diff --git a/extensions/browser/external_provider_interface.h b/extensions/browser/external_provider_interface.h
index 801f4ede5640de99259898da3c2b0e22cbb9e336..e52ac4546f5458c57b5e74bf04237aed4fb74ddf 100644
--- a/extensions/browser/external_provider_interface.h
+++ b/extensions/browser/external_provider_interface.h
@@ -52,7 +52,8 @@ class ExternalProviderInterface {
         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
@@ -62,6 +63,15 @@ class ExternalProviderInterface {
     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() {}
   };