Componentize about_flags::FeatureEntry in flags_ui component.

components/flags_ui.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.
 
 {
   'targets': [
     {
       # GN version: //components/flags_ui
       'target_name': 'flags_ui',
       'type': 'static_library',
       'include_dirs': [
         '..',
       ],
+      'dependencies': [
+        '../base/base.gyp:base',
+        '../base/base.gyp:base_prefs',
+        '../components/components_strings.gyp:components_strings',
+        '../ui/base/ui_base.gyp:ui_base',
+      ],
       'sources': [
         # Note: sources list duplicated in GN build.
+        'flags_ui/feature_entry.cc',
+        'flags_ui/feature_entry.h',
+        'flags_ui/feature_entry_macros.h',
         'flags_ui/flags_ui_constants.cc',
         'flags_ui/flags_ui_constants.h',
         'flags_ui/flags_ui_pref_names.cc',
         'flags_ui/flags_ui_pref_names.h',
         'flags_ui/flags_storage.h',
         'flags_ui/pref_service_flags_storage.cc',
         'flags_ui/pref_service_flags_storage.h',
       ],
     },
   ],
 }