[css-logical-props] logical property values on computed style objects

The spec should talk about how logical properties are exposed on 
computed style objects.  I think it would make sense to expose their 
logical values based on the direction/text-orientation/writing-mode 
values, for example:

   <style>
     div { direction: rtl; margin-right: 10px; }
   </style>
   <div></div>
   <script>
     var cs = getComputedStyle(document.querySelector("div"));
     alert(cs.getPropertyValue("margin-inline-start"));  // "10px"
   </script>