Add search_engines and precache components to GN build.

components/precache.gypi

 # Copyright 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/precache/content
       'target_name': 'precache_content',
       'type': 'static_library',
       'dependencies': [
         'precache_core',
         '../base/base.gyp:base',
         '../content/content.gyp:content_browser',
         '../url/url.gyp:url_lib',
       ],
       'include_dirs': [
         '..',
       ],
       'sources': [
+        # Note: sources list duplicated in GN build.
         'precache/content/precache_manager.cc',
         'precache/content/precache_manager.h',
         'precache/content/precache_manager_factory.cc',
         'precache/content/precache_manager_factory.h',
       ],
     },
     {
+      # GN version: //components/precache/core
       'target_name': 'precache_core',
       'type': 'static_library',
       'dependencies': [
         'precache_core_proto',
         '../base/base.gyp:base',
         '../third_party/protobuf/protobuf.gyp:protobuf_lite',
         '../url/url.gyp:url_lib',
       ],
       'include_dirs': [
         '..',
@@ skipping 10 matching lines @@
         'precache/core/url_list_provider.h',
       ],
       'includes': [ 'precache/precache_defines.gypi', ],
       'direct_dependent_settings': {
         # Make direct dependents also include the precache defines. This allows
         # the unit tests to use these defines.
         'includes': [ 'precache/precache_defines.gypi', ],
       },
     },
     {
+      # GN version: //components/precache/core:proto
       'target_name': 'precache_core_proto',
       'type': 'static_library',
       'sources': [
         'precache/core/proto/precache.proto',
       ],
       'variables': {
         'proto_in_dir': 'precache/core/proto',
         'proto_out_dir': 'components/precache/core/proto',
       },
       'includes': [ '../build/protoc.gypi', ],
     },
   ],
 }