Implement ::content

Source/core/css/CSSGrammar.y.in

Index: Source/core/css/CSSGrammar.y.in
diff --git a/Source/core/css/CSSGrammar.y.in b/Source/core/css/CSSGrammar.y.in
index b398b89a403e7d0e5475271af5d0478663815022..9d35693268926d475d1e58399873baab2dbbcf5d 100644
--- a/Source/core/css/CSSGrammar.y.in
+++ b/Source/core/css/CSSGrammar.y.in
@@ -1183,6 +1183,8 @@ selector:
         while (end->tagHistory())
             end = end->tagHistory();
         end->setRelation(CSSSelector::Descendant);
+        if ($1->isContentPseudoElement())
+            end->setRelationIsForShadowDistributed();
         end->setTagHistory(parser->sinkFloatingSelector($1));
     }
     | selector combinator simple_selector {
@@ -1191,6 +1193,8 @@ selector:
         while (end->tagHistory())
             end = end->tagHistory();
         end->setRelation($2);
+        if ($1->isContentPseudoElement())
+            end->setRelationIsForShadowDistributed();
         end->setTagHistory(parser->sinkFloatingSelector($1));
     }
     ;