Add search_engines and precache components to GN build.

components/search_engines/prepopulated_engines.gyp

 # Copyright 2014 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/search_engines:prepopulated_engines
       'target_name': 'prepopulated_engines',
       'type': 'static_library',
       'sources': [
         'prepopulated_engines.json',
       ],
       'includes': [
         '../../build/json_to_struct.gypi',
       ],
       'variables': {
         'chromium_code': 1,
         'schema_file': 'prepopulated_engines_schema.json',
         'namespace': 'TemplateURLPrepopulateData',
         'cc_dir': 'components/search_engines',
       },
     },
   ],
 }