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..e84afdf8e5cef45551a4e0dbb20c9af9da338ca7 100644
--- a/Source/core/css/CSSParserValues.h
+++ b/Source/core/css/CSSParserValues.h
@@ -193,6 +193,7 @@ 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); }