Move plugin placeholder style to CSS, and allow it to bypass main world CSP.

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 580 matching lines @@
               'css/themeWin.css',
               'css/themeWinQuirks.css',
               'css/svg.css',
               'css/navigationTransitions.css',
               'css/mathml.css',
               'css/mediaControls.css',
               'css/mediaControlsAndroid.css',
               'css/fullscreen.css',
               'css/xhtmlmp.css',
               'css/viewportAndroid.css',
+              'html/shadow/PluginPlaceholderElement.css',
             ],
           },
           'inputs': [
             '<@(scripts)',
             '<@(stylesheets)'
           ],
           'outputs': [
             '<(blink_core_output_dir)/UserAgentStyleSheets.h',
             '<(blink_core_output_dir)/UserAgentStyleSheetsData.cpp',
           ],
@@ skipping 219 matching lines @@
             '../build/scripts/rule_bison.py',
             '<(RULE_INPUT_PATH)',
             '<(blink_core_output_dir)',
             '<(bison_exe)',
           ],
         },
       ],
     },
   ],
 }