Fix assert on unknown rule type in the BisonCSSParser::endRuleHeader.

Source/core/css/CSSGrammar.y

 %{
 
 /*
  *  Copyright (C) 2002-2003 Lars Knoll (knoll@kde.org)
  *  Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc. All rights reserved.
  *  Copyright (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  *  Copyright (C) 2008 Eric Seidel <eric@webkit.org>
  *  Copyright (C) 2012 Intel Corporation. All rights reserved.
  *
  *  This library is free software; you can redistribute it and/or
@@ skipping 710 matching lines @@
 at_rule_header_end_maybe_space:
     maybe_space {
         parser->endRuleHeader();
     }
     ;
 
 media_rule_start:
     before_media_rule MEDIA_SYM maybe_space;
 
 media:
-    media_rule_start maybe_media_list at_rule_header_end '{' at_rule_body_start maybe_space block_rule_body closing_brace {
+    media_rule_start maybe_media_list '{' at_rule_header_end at_rule_body_start maybe_space block_rule_body closing_brace {
         $$ = parser->createMediaRule($2, $7);
     }
     ;
 
 medium:
   IDENT
   ;
 
 supports:
     before_supports_rule SUPPORTS_SYM maybe_space supports_condition at_supports_rule_header_end '{' at_rule_body_start maybe_space block_rule_body closing_brace {
@@ skipping 1081 matching lines @@
     ;
 
 rule_error_recovery:
     /* empty */
   | rule_error_recovery error
   | rule_error_recovery invalid_square_brackets_block
   | rule_error_recovery invalid_parentheses_block
     ;
 
 %%