Add metro driver and delegate_execute to GN build.

ui/metro_viewer/metro_viewer.gyp

 # Copyright (c) 2012 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.
 
 {
   'variables': {
     'chromium_code': 1,
   },
   'targets': [
     {
+      # GN version: //ui/metro_viewer
       'target_name': 'metro_viewer_messages',
       'type': 'static_library',
       'dependencies': [
         '../../base/base.gyp:base',
       ],
       'sources': [
         'ime_types.cc',
         'ime_types.h',
         'metro_viewer_message_generator.cc',
         'metro_viewer_message_generator.h',
         'metro_viewer_messages.h',
       ],
       'include_dirs': [
         '..',
       ],
     },
   ],
 }