Allow "::content /deep/" to match.

Source/core/css/SelectorChecker.cpp

Index: Source/core/css/SelectorChecker.cpp
diff --git a/Source/core/css/SelectorChecker.cpp b/Source/core/css/SelectorChecker.cpp
index cff70da5bf3d5b3080ae8f813134c603f44ac2aa..3700398372d804cfed902f4923e74603d43b7a0b 100644
--- a/Source/core/css/SelectorChecker.cpp
+++ b/Source/core/css/SelectorChecker.cpp
@@ -350,6 +350,14 @@ SelectorChecker::Match SelectorChecker::matchForRelation(const SelectorCheckingC
 
     case CSSSelector::ShadowDeep:
         {
+            if (context.selector->relationIsAffectedByPseudoContent()) {
+                for (Element* element = context.element; element; element = parentOrShadowHostButDisallowEscapingClosedShadowTree(*element)) {
+                    if (matchForShadowDistributed(element, siblingTraversalStrategy, nextContext, result) == SelectorMatches)
+                        return SelectorMatches;
+                }
+                return SelectorFailsCompletely;
+            }
+
             nextContext.isSubSelector = false;
             nextContext.elementStyle = 0;
             for (nextContext.element = parentOrShadowHostButDisallowEscapingClosedShadowTree(*context.element); nextContext.element; nextContext.element = parentOrShadowHostButDisallowEscapingClosedShadowTree(*nextContext.element)) {