Switching CSS Variables over to RuntimeEnabledFeatures

LayoutTests/fast/css/variables/redefinition.html

-<script>
-internals.settings.setCSSVariablesEnabled(true);
-</script>
 <style>
 body {
     -webkit-var-a: red;
 }
 #a {
     -webkit-var-a: green;
 }
 #b {
     -webkit-var-a: blue;
 }
 div {
     color: -webkit-var(a);
 }
 </style>
 <div>
 This text should be red.
 </div>
 <div id="a">
 This text should be green.
 </div>
 <div id="b">
 This text should be blue.
 </div>