Implement ::shadow pseudo element and replace /shadow/ with ::shadow.

Source/core/css/RuleFeature.cpp

Index: Source/core/css/RuleFeature.cpp
diff --git a/Source/core/css/RuleFeature.cpp b/Source/core/css/RuleFeature.cpp
index a30022f1d0006737cbe311a7432ad9c3eba843c3..109f9723cfedf94b13d3ba0c7284866dc28a59e2 100644
--- a/Source/core/css/RuleFeature.cpp
+++ b/Source/core/css/RuleFeature.cpp
@@ -51,6 +51,7 @@ static bool isSkippableComponentForInvalidation(const CSSSelector& selector)
         case CSSSelector::PseudoBefore:
         case CSSSelector::PseudoAfter:
         case CSSSelector::PseudoBackdrop:
+        case CSSSelector::PseudoShadow:
             return true;
         default:
             return false;
@@ -118,7 +119,7 @@ RuleFeatureSet::InvalidationSetMode RuleFeatureSet::supportsClassDescendantInval
         switch (component->relation()) {
         case CSSSelector::Descendant:
         case CSSSelector::Child:
-        case CSSSelector::Shadow:
+        case CSSSelector::ShadowPseudo:
         case CSSSelector::ShadowDeep:
             foundDescendantRelation = true;
             // Fall through!