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

Source/core/css/SelectorFilter.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 2004-2005 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2006, 2007 Nicholas Shanks (webkit@nickshanks.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  * Copyright (C) 2007, 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (c) 2011, Code Aurora Forum. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011. All rights reserved.
@@ skipping 117 matching lines @@
     default:
         break;
     }
 }
 
 void SelectorFilter::collectIdentifierHashes(const CSSSelector* selector, unsigned* identifierHashes, unsigned maximumIdentifierCount)
 {
     unsigned* hash = identifierHashes;
     unsigned* end = identifierHashes + maximumIdentifierCount;
     CSSSelector::Relation relation = selector->relation();
+    bool relationIsForShadowDistributed = selector->relationIsForShadowDistributed();
 
     // Skip the topmost selector. It is handled quickly by the rule hashes.
     bool skipOverSubselectors = true;
     for (selector = selector->tagHistory(); selector; selector = selector->tagHistory()) {
         // Only collect identifiers that match ancestors.
         switch (relation) {
         case CSSSelector::SubSelector:
             if (!skipOverSubselectors)
                 collectDescendantSelectorIdentifierHashes(selector, hash);
             break;
         case CSSSelector::DirectAdjacent:
         case CSSSelector::IndirectAdjacent:
         case CSSSelector::ShadowPseudo:
-        case CSSSelector::ShadowDistributed:
             skipOverSubselectors = true;
             break;
         case CSSSelector::Descendant:
         case CSSSelector::Child:
+            if (relationIsForShadowDistributed) {
+                skipOverSubselectors = true;
+                break;
+            }
             skipOverSubselectors = false;
             collectDescendantSelectorIdentifierHashes(selector, hash);
             break;
         }
         if (hash == end)
             return;
         relation = selector->relation();
+        relationIsForShadowDistributed = selector->relationIsForShadowDistributed();
     }
     *hash = 0;
 }
 
 }