Add search_engines and precache components to GN build.

components/search_engines.gypi

 # 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
       'target_name': 'search_engines',
       'type': 'static_library',
       'dependencies': [
         '../base/base.gyp:base',
         '../google_apis/google_apis.gyp:google_apis',
         '../net/net.gyp:net',
         '../sql/sql.gyp:sql',
         '../sync/sync.gyp:sync',
         '../third_party/libxml/libxml.gyp:libxml',
         '../ui/gfx/gfx.gyp:gfx',
         '../url/url.gyp:url_lib',
         'component_metrics_proto',
         'components_strings.gyp:components_strings',
         'google_core_browser',
         'keyed_service_core',
         'policy',
         'pref_registry',
         'rappor',
         'search_engines/prepopulated_engines.gyp:prepopulated_engines',
         'url_fixer',
         'webdata_common',
       ],
       'include_dirs': [
         '..',
       ],
       'sources': [
+        # Note: sources list duplicated in GN build.
         'search_engines/default_search_manager.cc',
         'search_engines/default_search_manager.h',
         'search_engines/default_search_policy_handler.cc',
         'search_engines/default_search_policy_handler.h',
         'search_engines/keyword_table.cc',
         'search_engines/keyword_table.h',
         'search_engines/keyword_web_data_service.cc',
         'search_engines/keyword_web_data_service.h',
         'search_engines/search_engine_type.h',
         'search_engines/search_engines_pref_names.cc',
@@ skipping 18 matching lines @@
         'search_engines/template_url_service.cc',
         'search_engines/template_url_service.h',
         'search_engines/template_url_service_client.h',
         'search_engines/template_url_service_observer.h',
         'search_engines/util.cc',
         'search_engines/util.h',
       ],
     },
   ],
 }