Add a custom element to own structure of shadow DOM plugin placeholders.

Source/core/core_generated.gyp

 #
 # Copyright (C) 2013 Google Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
@@ skipping 133 matching lines @@
       ],
       'actions': [
         {
           'action_name': 'generatePrivateScript',
           # FIXME: The implementation of Blink-in-JS is not yet mature.
           # You can use Blink-in-JS in your local experiment, but don't ship it.
           # crbug.com/341031
           'private_script_files': [
              '../bindings/core/v8/PrivateScriptRunner.js',
              'html/HTMLMarqueeElement.js',
+             'html/shadow/PluginPlaceholderElement.js',
              'xml/DocumentXMLTreeViewer.js',
           ],
           'inputs': [
              '../build/scripts/make_private_script_source.py',
              '<@(_private_script_files)',
           ],
           'outputs': [
             '<(blink_core_output_dir)/PrivateScriptSources.h',
           ],
           'action': [
@@ skipping 644 matching lines @@
             '../build/scripts/rule_bison.py',
             '<(RULE_INPUT_PATH)',
             '<(blink_core_output_dir)',
             '<(bison_exe)',
           ],
         },
       ],
     },
   ],
 }