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

Source/core/css/SelectorFilter.cpp

Index: Source/core/css/SelectorFilter.cpp
diff --git a/Source/core/css/SelectorFilter.cpp b/Source/core/css/SelectorFilter.cpp
index aedefed921e6c86a8825e2a4e13c8cead6813c42..632a7ec3f4023999d4a3655c0743fabdde6c1c01 100644
--- a/Source/core/css/SelectorFilter.cpp
+++ b/Source/core/css/SelectorFilter.cpp
@@ -135,6 +135,7 @@ void SelectorFilter::collectIdentifierHashes(const CSSSelector* selector, unsign
     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;
@@ -148,11 +149,14 @@ void SelectorFilter::collectIdentifierHashes(const CSSSelector* selector, unsign
         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;
@@ -160,6 +164,7 @@ void SelectorFilter::collectIdentifierHashes(const CSSSelector* selector, unsign
         if (hash == end)
             return;
         relation = selector->relation();
+        relationIsForShadowDistributed = selector->relationIsForShadowDistributed();
     }
     *hash = 0;
 }