Add chrome test support target to GN build.

components/pref_registry.gypi

 # Copyright (c) 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.
 
 {
   'targets': [
     {
+      # GN version: //components/pref_registry
       'target_name': 'pref_registry',
       'type': '<(component)',
       'dependencies': [
         '../base/base.gyp:base',
         '../base/base.gyp:base_prefs',
         '../base/third_party/dynamic_annotations/dynamic_annotations.gyp:dynamic_annotations',
         '../ui/base/ui_base.gyp:ui_base',
       ],
       'include_dirs': [
         '..',
       ],
       'defines': [
         'PREF_REGISTRY_IMPLEMENTATION',
       ],
       'sources': [
         'pref_registry/pref_registry_export.h',
         'pref_registry/pref_registry_syncable.cc',
         'pref_registry/pref_registry_syncable.h',
       ],
     },
     {
+      # GN version: //components/pref_registry:test_support
       'target_name': 'pref_registry_test_support',
       'type': 'static_library',
       'dependencies': [
         'pref_registry',
         '../base/base.gyp:base_prefs_test_support',
       ],
       'include_dirs': [
         '..',
       ],
       'sources': [
         'pref_registry/testing_pref_service_syncable.cc',
         'pref_registry/testing_pref_service_syncable.h',
       ],
     },
   ],
 }