Add support for app_shell-only extension APIs

extensions/shell/app_shell_resources.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': [
     {
-      'target_name': 'athena_strings',
+      'target_name': 'app_shell_resources',
       'type': 'none',
       'variables': {
-        'grit_out_dir': '<(SHARED_INTERMEDIATE_DIR)/athena/strings',
+        'grit_out_dir': '<(SHARED_INTERMEDIATE_DIR)/extensions/shell',
       },
       'actions': [
         {
-          'action_name': 'generate_athena_strings',
+          'action_name': 'generate_app_shell_resources',
           'variables': {
-            'grit_grd_file': 'athena_strings.grd',
+            'grit_grd_file': 'app_shell_resources.grd',
           },
           'includes': [ '../../build/grit_action.gypi' ],
         },
       ],
+      'includes': [ '../../build/grit_target.gypi' ],
     },
-  ],
+  ]  # targets
 }