Only update composited-scrolling state once after layout.

LayoutTests/compositing/overflow/resources/build-paint-order-lists.js

 function getPaintOrder(element)
 {
   var divElementsBeforePromote = [];
   var divElementsAfterPromote = [];
   // Force a style recalc.
+  document.body.style.overflow = 'scroll';
+  document.body.offsetTop;
+  document.body.style.overflow = '';
   document.body.offsetTop;
 
   var paintOrderListBeforePromote = window.internals.paintOrderListBeforePromote(element);
   var paintOrderListAfterPromote = window.internals.paintOrderListAfterPromote(element);
 
   for (var i = 0; i < paintOrderListBeforePromote.length; ++i)
     if (paintOrderListBeforePromote[i].nodeName === "DIV")
       divElementsBeforePromote.push(paintOrderListBeforePromote[i]);
 
   for (var i = 0; i < paintOrderListAfterPromote.length; ++i)
@@ skipping 17 matching lines @@
 }
 
 function countOccurrencesOfElementInPaintOrderList(paintOrder, element) {
   var occurrenceCount = 0;
   for (var i = 0; i < paintOrder.length; i++)
     if (paintOrder[i] === element)
       occurrenceCount++;
 
   return occurrenceCount;
 }