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

Source/core/css/CSSParser.cpp

 /*
  * Copyright (C) 2003 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Nicholas Shanks <webkit@nickshanks.com>
  * Copyright (C) 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2012 Adobe Systems Incorporated. All rights reserved.
  * Copyright (C) 2012 Intel Corporation. All rights reserved.
  *
@@ skipping 11120 matching lines @@
     ASSERT(!specifiers->isDistributedPseudoElement());
     for (CSSParserSelector* end = specifiers; end->tagHistory(); end = end->tagHistory()) {
         if (end->tagHistory()->isDistributedPseudoElement()) {
             end->clearTagHistory();
             break;
         }
     }
     CSSParserSelector* end = argumentSelector;
     while (end->tagHistory())
         end = end->tagHistory();
-    end->setTagHistory(sinkFloatingSelector(specifiers));
-    end->setRelation(CSSSelector::ShadowDistributed);
-    return argumentSelector;
+
+    switch (end->relation()) {
+    case CSSSelector::Child:
+    case CSSSelector::Descendant:
+        end->setTagHistory(sinkFloatingSelector(specifiers));
+        end->setRelationIsForShadowDistributed();
+        return argumentSelector;
+    default:
+        return 0;
+    }
 }
 
 CSSParserSelector* CSSParser::rewriteSpecifiers(CSSParserSelector* specifiers, CSSParserSelector* newSpecifier)
 {
     if (newSpecifier->isCustomPseudoElement() || newSpecifier->pseudoType() == CSSSelector::PseudoCue) {
         // Unknown pseudo element always goes at the top of selector chain.
         newSpecifier->appendTagHistory(CSSSelector::ShadowPseudo, sinkFloatingSelector(specifiers));
         return newSpecifier;
     }
     if (specifiers->isCustomPseudoElement()) {
@@ skipping 557 matching lines @@
     result.lineNumber = lineNumber;
     result.content = content;
     size_t newLength = content.length();
     while (newLength > 0 && isHTMLSpace(result.content[newLength - 1]))
         --newLength;
     result.content.setLength(newLength);
     return result;
 }
 
 }