Add forEach() to CSSVariablesMap

Source/core/css/CSSVariablesMapForEachCallback.idl

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-interface TestInterfaceEmpty {
+callback interface CSSVariablesMapForEachCallback {
+    [CallWith=ThisValue] boolean handleItem(DOMString value, DOMString name, CSSVariablesMap map);
+    boolean handleItem(DOMString value, DOMString name, CSSVariablesMap map);
 };