Observe adding external extensions via windows registry.

extensions/extensions.gypi

 # Copyright 2015 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.
 
 {
   'variables': {
     'extensions_common_constants_sources': [
       'common/constants.cc',
       'common/constants.h',
     ],
@@ skipping 618 matching lines @@
       'browser/extension_user_script_loader.cc',
       'browser/extension_user_script_loader.h',
       'browser/extension_util.cc',
       'browser/extension_util.h',
       'browser/extension_web_contents_observer.cc',
       'browser/extension_web_contents_observer.h',
       'browser/extension_zoom_request_client.cc',
       'browser/extension_zoom_request_client.h',
       'browser/extensions_browser_client.cc',
       'browser/extensions_browser_client.h',
+      'browser/external_install_info.cc',
+      'browser/external_install_info.h',
       'browser/external_provider_interface.h',
       'browser/file_highlighter.cc',
       'browser/file_highlighter.h',
       'browser/file_reader.cc',
       'browser/file_reader.h',
       'browser/granted_file_entry.cc',
       'browser/granted_file_entry.h',
       'browser/guest_view/app_view/app_view_constants.cc',
       'browser/guest_view/app_view/app_view_constants.h',
       'browser/guest_view/app_view/app_view_guest.cc',
@@ skipping 441 matching lines @@
       'test/result_catcher.h',
       'test/test_content_utility_client.cc',
       'test/test_content_utility_client.h',
       'test/test_extensions_client.cc',
       'test/test_extensions_client.h',
       'test/test_permission_message_provider.cc',
       'test/test_permission_message_provider.h',
     ],
   },
 }