Make a '::distributed' pseudo element the first-ever client who can accept a relative selector.

Source/core/css/CSSGrammar.y.in

 /*
  *  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
  *  modify it under the terms of the GNU Lesser General Public
  *  License as published by the Free Software Foundation; either
@@ skipping 69 matching lines @@
     case VAR_DEFINITION:
     case UNICODERANGE:
         return true;
     default:
         return false;
     }
 }
 
 %}
 
-%expect 28
+%expect 29
 
 %nonassoc LOWEST_PREC
 
 %left UNIMPORTANT_TOK
 
 %token WHITESPACE SGML_CD
 %token TOKEN_EOF 0
 
 %token INCLUDES
 %token DASHMATCH
@@ skipping 168 matching lines @@
 %type <keyframeRuleList> keyframes_rule
 %type <valueList> key_list
 %type <value> key
 
 %type <id> property
 
 %type <selector> specifier
 %type <selector> specifier_list
 %type <selector> simple_selector
 %type <selector> selector
+%type <selector> relative_selector
 %type <selectorList> selector_list
 %type <selectorList> simple_selector_list
 %type <selectorList> region_selector
 %type <selector> selector_with_trailing_whitespace
 %type <selector> class
 %type <selector> attrib
 %type <selector> pseudo
 %type <selector> pseudo_page
 %type <selector> page_selector
 
@@ skipping 829 matching lines @@
     | selector_list at_selector_end ',' maybe_space before_selector_group_item selector %prec UNIMPORTANT_TOK {
         if ($1 && $6) {
             $$ = $1;
             $$->append(parser->sinkFloatingSelector($6));
             parser->updateLastSelectorLineAndPosition();
         } else
             $$ = 0;
     }
    ;
 
+relative_selector:
+    combinator selector {
+        $$ = $2;
+        CSSParserSelector* end = $$;
+        while (end->tagHistory())
+            end = end->tagHistory();
+        end->setRelation($1);
+    }
+    | selector {
+        $$ = $1;
+    }
+    ;
+
 selector_with_trailing_whitespace:
     selector WHITESPACE {
         $$ = $1;
     }
     ;
 
 selector:
     simple_selector {
         $$ = $1;
     }
@@ skipping 240 matching lines @@
             $$->setValue($3);
             CSSSelector::PseudoType type = $$->pseudoType();
             if (type != CSSSelector::PseudoCue)
                 $$ = 0;
         } else
             $$ = 0;
     }
     | ':' ':' CUEFUNCTION selector_recovery ')' {
         YYERROR;
     }
-    | ':' ':' DISTRIBUTEDFUNCTION maybe_space selector maybe_space ')' {
+    | ':' ':' DISTRIBUTEDFUNCTION maybe_space relative_selector maybe_space ')' {
         if (!$5)
             $$ = 0;
         else {
             $$ = parser->createFloatingSelector();
             $$->setMatch(CSSSelector::PseudoElement);
             $$->setFunctionArgumentSelector($5);
             parser->tokenToLowerCase($3);
             $$->setValue($3);
         }
     }
@@ skipping 550 matching lines @@
 
 rule_error_recovery:
     /* empty */
   | rule_error_recovery error
   | rule_error_recovery invalid_square_brackets_block
   | rule_error_recovery invalid_parentheses_block
     ;
 
 %%