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"
+#include "extensions/browser/external_extension_install_info.h"
 #include "extensions/common/manifest.h"
 
 class GURL;
 
 namespace base {
-class FilePath;
 class Version;
 }
 
 namespace extensions {
 
 // This class is an abstract class for implementing external extensions
 // providers.
 class ExternalProviderInterface {
  public:
   // ExternalProvider uses this interface to communicate back to the
   // caller what extensions are registered, and which |id|, |version| and |path|
   // they have. See also VisitRegisteredExtension below. Ownership of |version|
   // is not transferred to the visitor.  Callers of the methods below must
   // ensure that |id| is a valid extension id (use
   // crx_file::id_util::IdIsValid(id)).
   class VisitorInterface {
    public:
     // Return true if the extension install will proceed.  Install will not
     // proceed if the extension is already installed from a higher priority
     // location.
     virtual bool OnExternalExtensionFileFound(
-        const std::string& id,
-        const base::Version* version,
-        const base::FilePath& path,
-        Manifest::Location location,
-        int creation_flags,
-        bool mark_acknowledged,
-        bool install_immediately) = 0;
+        ExternalExtensionInstallInfoFile* info) = 0;

[asargent_no_longer_on_chrome, 2016/01/25 18:50:52]

nit: If ownership of this data is passed, this should probably use
scoped_ptr<ExternalExtensionInstallInfoFile> to make that clear. If ownership
isn't passed, it should use const ExternalExtensionInstallInfoFile& (I assume it
wouldn't be useful to be able to pass nullptr)

[lazyboy, 2016/01/26 05:20:05]

Done.

 
     // 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;
+        ExternalExtensionInstallInfoUpdateUrl* info,

[asargent_no_longer_on_chrome, 2016/01/25 18:50:52]

same comment re: ownership

[lazyboy, 2016/01/26 05:20:05]

Done.

+        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 ScopedVector<ExternalExtensionInstallInfoUpdateUrl>&
+            update_url_extensions,
+        const ScopedVector<ExternalExtensionInstallInfoFile>& file_extensions,
+        const std::set<std::string>& removed_extensions) = 0;
+
    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_