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

Source/core/css/CSSParserValues.h

Index: Source/core/css/CSSParserValues.h
diff --git a/Source/core/css/CSSParserValues.h b/Source/core/css/CSSParserValues.h
index fd1c475a3f3254d4e0da2f1e125d4fd9a97f1e8e..7e7079e527317f88a348fd7458951e015ac97ab7 100644
--- a/Source/core/css/CSSParserValues.h
+++ b/Source/core/css/CSSParserValues.h
@@ -193,12 +193,14 @@ public:
 
     PassOwnPtr<CSSSelector> releaseSelector() { return m_selector.release(); }
 
+    CSSSelector::Relation relation() const { return m_selector->relation(); }
     void setValue(const AtomicString& value) { m_selector->setValue(value); }
     void setAttribute(const QualifiedName& value) { m_selector->setAttribute(value); }
     void setArgument(const AtomicString& value) { m_selector->setArgument(value); }
     void setMatch(CSSSelector::Match value) { m_selector->m_match = value; }
     void setRelation(CSSSelector::Relation value) { m_selector->m_relation = value; }
     void setForPage() { m_selector->setForPage(); }
+    void setRelationIsForShadowDistributed() { m_selector->setRelationIsForShadowDistributed(); }
 
     void adoptSelectorVector(Vector<OwnPtr<CSSParserSelector> >& selectorVector);