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

Source/core/css/CSSGrammar.y

Index: Source/core/css/CSSGrammar.y
diff --git a/Source/core/css/CSSGrammar.y b/Source/core/css/CSSGrammar.y
index 32e44cd47b79df9261199fcdd8b678436deccdbd..8aeb5f8a834d1562a11a184984a1f06b52ddd6b0 100644
--- a/Source/core/css/CSSGrammar.y
+++ b/Source/core/css/CSSGrammar.y
@@ -1046,12 +1046,8 @@ combinator:
     '+' maybe_space { $$ = CSSSelector::DirectAdjacent; }
     | '~' maybe_space { $$ = CSSSelector::IndirectAdjacent; }
     | '>' maybe_space { $$ = CSSSelector::Child; }
-    // FIXME: implement named combinator and replace the following /shadow/, /shadow-child/ and
-    // /shadow-deep/ with named combinator's implementation.
     | '/' IDENT '/' maybe_space {
-        if ($2.equalIgnoringCase("shadow"))
-            $$ = CSSSelector::Shadow;
-        else if ($2.equalIgnoringCase("shadow-deep"))
+        if ($2.equalIgnoringCase("shadow-deep"))
             $$ = CSSSelector::ShadowDeep;
         else
             YYERROR;