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

Source/core/css/CSSSelector.cpp

Index: Source/core/css/CSSSelector.cpp
diff --git a/Source/core/css/CSSSelector.cpp b/Source/core/css/CSSSelector.cpp
index b02150d117e219f51761c14f931a6c5bcb8e92b8..e60b346c6859a4739aab5b5ce411c887e2741612 100644
--- a/Source/core/css/CSSSelector.cpp
+++ b/Source/core/css/CSSSelector.cpp
@@ -255,6 +255,7 @@ PseudoId CSSSelector::pseudoId(PseudoType type)
     case PseudoContent:
     case PseudoHost:
     case PseudoAncestor:
+    case PseudoShadow:
     case PseudoFullScreen:
     case PseudoFullScreenDocument:
     case PseudoFullScreenAncestor:
@@ -358,6 +359,7 @@ const static NameToPseudoStruct pseudoTypeMap[] = {
 {"ancestor",                      CSSSelector::PseudoAncestor,            CSSSelector::RequiresShadowDOM},
 {"ancestor(",                     CSSSelector::PseudoAncestor,            CSSSelector::RequiresShadowDOM},
 {"content",                       CSSSelector::PseudoContent,             CSSSelector::RequiresShadowDOM},
+{"shadow",                        CSSSelector::PseudoShadow,              CSSSelector::RequiresShadowDOM},
 };
 
 static HashMap<StringImpl*, CSSSelector::PseudoType>* nameToPseudoTypeMap()
@@ -463,6 +465,7 @@ void CSSSelector::extractPseudoType() const
     case PseudoUserAgentCustomElement:
     case PseudoWebKitCustomElement:
     case PseudoContent:
+    case PseudoShadow:
         element = true;
         break;
     case PseudoUnknown:
@@ -703,8 +706,6 @@ String CSSSelector::selectorText(const String& rightSide) const
             if (cs->relationIsAffectedByPseudoContent() && tagHistory->pseudoType() != CSSSelector::PseudoContent)
                 return tagHistory->selectorText("::-webkit-distributed(> " + str.toString() + rightSide + ")");
             return tagHistory->selectorText(" > " + str.toString() + rightSide);
-        case CSSSelector::Shadow:
-            return tagHistory->selectorText(" /shadow/ " + str.toString() + rightSide);
         case CSSSelector::ShadowDeep:
             return tagHistory->selectorText(" /shadow-deep/ " + str.toString() + rightSide);
         case CSSSelector::DirectAdjacent: